EL display apparatus, driving circuit of EL display apparatus, and image display apparatus

ABSTRACT

The EL display apparatus according to this invention is provided with an EL light emitting element, a current driving device for driving the EL light emitting element by a current responsive to a source signal represented by a current, and a signal current source ( 634 ) for outputting the source signal in response to image signal to the current driving device via a source signal line, the EL display apparatus being further provided with a precharge voltage source ( 631 ) for outputting a predetermined voltage and a switching and connecting unit ( 636, 637 ) capable of selectively connecting either the signal current source or the precharge voltage source to the source signal line ( 638 ).

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/488,766, filed Sep. 30, 2004 now abandoned, that is based on a 371 ofInternational Application No. PCT/JP2002/09112, filed Sep. 6, 2002,which in turn claims the benefit of Japanese Patent Application Nos.2001-271311 filed on Jul. 9, 2001 and 2001-347014 filed on Nov. 13,2001, and the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a light emitting display panel such asan EL display panel which uses an organic or inorganicelectroluminescence (EL) element. Further, the invention relates to anEL display panel driving method, an EL display panel driving circuit,and an electronic display appliance using the method and the circuit.

BACKGROUND ART

In general, an active-matrix display apparatus has a multiplicity ofpixels arranged in matrix and displays an image by controlling theintensity of light pixel by pixel in accordance with image signalsgiven. When, for example, liquid crystal is used as an electro-opticsubstance, the transmittance of each pixel varies in accordance with thevoltage applied to the pixel. The basic operation of an active-matriximage display apparatus employing an organic electroluminescence (EL)material as an electro-optic converting substance is the same as in thecase where liquid crystal is used.

A liquid crystal display panel has pixels each functioning as a shutterand displays an image by turning on/off light from a back light withsuch a shutter, or a pixel. An organic EL display panel is a displaypanel of the self-luminescence type having a light-emitting device ineach pixel. Such a self-luminescence type display panel has advantagesover liquid crystal display panels, including higher image visibility,no need for a back light, and higher response speed.

The organic EL display panel controls the luminance of eachlight-emitting device (pixel) based on the amount of current. Thus, theorganic EL display panel is largely different from the liquid crystaldisplay panel in that its luminescent devices are of the current-driventype or the current-controlled type.

Like the liquid crystal display panel, the organic EL display panel canhave any one of a simple-matrix configuration and an active-matrixconfiguration. Though the former configuration is simple in structure,it has a difficulty in realizing a large-scale and high-definitiondisplay panel. However, it is inexpensive. The latter configuration canrealize a large-scale and high-definition display panel. However, it hasproblems of a technical difficulty in control and of a relatively highprice. Presently, organic EL display panels of the active-matrixconfiguration are being developed intensively. Such an active-matrix ELpanel controls electric current passing through the light-emittingdevice provided in each pixel by means of a thin film transistor (TFT)located inside the pixel.

This active matrix type organic display panel is disclosed in JapaneseUnexamined Patent Publication No. 8-234683. An equivalent circuit forone pixel of the display panel is shown in FIG. 62. A pixel 16 isprovided with an EL element 15 serving as a light emitting element, afirst transistor 11 a, a second transistor 11 b, and capacitance 19. Thelight emitting element 15 is an organic electroluminescence (EL)element. In this invention, the transistor 11 a which supplies(controls) a current to the EL element 15 is referred to as a drivingtransistor 11. Also, a transistor which operates as a switch, such asthe transistor 11 b of FIG. 62, is referred to as a switching transistor11.

Since the organic EL element 15, in general, has rectification property,it is called OLED (organic light emitting diode) in some cases. In FIG.62, the EL element is denoted by OLED 15 of which D indicates diode.

Note that the light emitting element 15 of this invention is not limitedto the OLED, and other light emitting diodes are usable so far as aluminance thereof is controlled by adjusting an amount of a currentsupplied thereto. For example, an inorganic EL element is also usable.Another example may be a white light emitting diode made from asemiconductor. Yet another example may be general light emitting diodes.A light emitting transistor may also be usable. The light emitting diode15 does not necessarily show the rectification property. A bidirectionaldiode may be used as the light emitting diode 15.

In the example shown in FIG. 62, the source terminal (S) of p-channeltransistor 11 a is connected to Vdd (power source potential), while thecathode (negative electrode) of the EL device 15 connected to groundpotential (Vk). On the other hand, the anode (positive electrode) isconnected to the drain terminal (D) of the transistor 11 b. The gateterminal of the p-channel transistor 11 b is connected to a gate signalline 17 a, the source terminal connected to a source signal line 18, andthe drain terminal connected to the storage capacitor 19 and the gateterminal (G) of the transistor 21 a.

In order to operate the pixel 16, first, the source signal line 18 isapplied with an image signal indicative of luminance information withthe gate signal line 17 a turned into a selected state. Then, thetransistor 11 b becomes conducting and the storage capacitor 19 ischarged or discharged, so that the gate potential of the transistor 11 abecomes equal to the potential of the image signal. When the gate signalline 17 a is turned into an unselected state, the transistor 11 a isturned off, so that the transistor 11 a is electrically disconnectedfrom the source signal line 18. However, the gate potential of thetransistor 11 a is stably maintained by means of the storage capacitor19. The current passing through the EL device 15 via the transistor 11 acomes to assume a value corresponding to voltage Vgs across the gate andthe source terminals of the transistor 11 a, with the result that the ELdevice 15 keeps on emitting light at a luminance corresponding to theamount of current fed thereto through the transistor 11 a.

As described above, according to the prior art configuration shown inFIG. 62, one pixel comprises one selecting transistor (switching device)and one driving transistor. Another prior art configuration is disclosedin Japanese Patent Laid-Open Publication No. HEI 11-327637 for example.This publication describes an embodiment in which a pixel comprises acurrent mirror circuit.

With the method shown in FIG. 62 of outputting an image signal as avoltage from a source driver 14, an output stage impedance of the sourcedriver 14 is low. Therefore, it is easy to program the image signal tothe source signal line 18.

With the method of outputting an image signal as a current, such as acurrent mirror structure shown in FIG. 1 or disclosed in Japanese PatentApplication No. 11-327637, an output stage impedance of a source driver14 is high. Therefore, it is undesirably difficult to program the imagesignal to the source signal line 18 in a black display region. FIG. 2 isan illustration of a reason for the difficulty.

In order to cause the light emitting element 15 of each of the pixels 16of FIG. 2 to display, the transistors 11 b and 11 c are brought into theconductive state by the gate signal line 17 a in one horizontal scanperiod, so that a current Iw is drawn from the power source Vdd to thesource driver 14 via the driving transistor 11 a and the source signalline 18. Gradation display is performed in accordance with an amount ofthe current drawn to the source driver 14. A charge responsive to thegate voltage corresponding to the drain current of the transistor 11 ais accumulated in the capacitance 19.

Then, the transistor 11 d is brought into the conductive state by thegate signal line 17 b, and the transistors 11 b and 11 c are broughtinto the non-conductive state by the gate signal line 17 a, whereby acurrent responsive to the charge in the capacitance 19 flows from theVdd to the light emitting element 15 via the transistor 11 a.

The current flowing to the source signal line 18 changes graduallydepending on a product of stray capacity (stray capacity) 641 of thesource signal line 18 and source-drain (S-D) resistance of thetransistor 12. Therefore, when the capacitance 641 and the resistanceare increased too much, the current sometimes fails to reach apredetermined value in one horizontal scan period.

With a reduction in the current flowing to the source signal line 18 (inthe case of low gray scale), the source-drain resistance of thetransistor 11 a is increased; therefore, time required for the currentto change is increased with the reduction in the current. Though itdepends on diode characteristics of the transistor 11 a and a value ofthe stray capacity 641, it takes 50μ seconds for the current flowing tothe source signal line 18 to change to 1 μA, and it takes 250μ secondsfor the current to change to 10 nA, for example.

The current flowing to the source signal line 18 supplies a charge tothe source signal line 18 from the Vdd via the transistor 12 a to changethe charge of the stray capacity 641, so that a voltage of the sourcesignal line 18 is changed to change the current flowing through thetransistor 12 a (the current flowing to the source signal line 18).Since a quantity of the supplied charge is small in a region where thecurrent is small, the voltage change on the source signal line 18 isslowed down to delay the change in the current.

Thus, it has been impossible to reduce the horizontal scan period, and,depending on the number of display columns, flickering occurs due to thereduction in frame frequency.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished in view of the aboveproblems, and an object thereof is to prevent occurrence of flickeringwhich is otherwise caused by a reduction in frame frequency.

In order to achieve the above object, an EL display apparatus accordingto this invention comprises an EL light emitting element, a currentdriving device for driving the EL light emitting element by a currentresponsive to a source signal represented by a current, and a signalcurrent source for outputting the source signal to the current drivingdevice through a source signal line in response to an image signal,characterized in that the EL display apparatus further comprises aprecharge voltage source for outputting a predetermined voltage and aswitching and connecting unit capable of selectively connecting eitherthe signal current source or the precharge voltage source to the sourcesignal line.

With such constitution, not only a source signal current is output tothe source signal line but also a precharge voltage is applied to thesource signal line when a current for low gray scale for whichprogramming is the most difficult is flowing to the source signal line.As a result, it is possible to charge stray capacity of the sourcesignal line rapidly by the use of a power source having low outputimpedance, thereby increasing a speed of change in current of thecurrent driving device. Thus, a horizontal scan period is reduced andflickering otherwise caused by a reduction in frame frequency isprevented.

The switching and connecting unit may connect the precharge voltagesource and the signal voltage source to the source signal such that thesource signal is output to the source signal line after thepredetermined voltage is applied to the source signal line in onehorizontal scan period. With such constitution, the stray capacity ofthe source signal line is rapidly charged to increase the speed of thechange in current of the current driving device.

A duration of applying the predetermined voltage may preferably be 0.2μseconds or more and 3μ seconds or less. With such constitution, thespeed of the change in current of the current driving device isfavorably increased.

The current driving device may drive the EL light emitting element by acurrent responsive to a voltage of a control terminal connected to thesource signal line, and the predetermined voltage may be set to a valueby which the current driving device so drives the EL light emittingelement as to achieve black display. With such constitution, the speedof the change in current of the current driving device at the time ofthe low gray scale is effectively increased.

Alternatively, the current driving device may drive the EL lightemitting element by a current responsive to a voltage of a controlterminal connected to the source signal line, and the predeterminedvoltage may be set to a value responsive to gray scale data of the imagesignal. With such constitution, an amount of a gray scale to be adjustedby the source signal is reduced to more rapidly change the current ofthe current driving device.

The switching and connecting unit may connect the precharge voltagesource to the source signal line when the gray scale data of the imagesignal is a predetermined one. With such constitution, a reduction inluminance is prevented by refraining from applying the precharge voltageat the time of high gray scale where the change in current of thecurrent driving device is rapid and when identical gray scale isrepeated.

A plurality of El light emitting elements for emitting light of pluralcolors may be connected respectively to the source signal lines for thecolors of light, and the precharge voltage source may output thepredetermined signal which is set for each of the colors of light toeach of the source signal lines. Transition time voltages of the ELlight emitting elements are varied depending on the colors of light,but, with such constitution, the precharge voltage optimum for the colorof light is applied so as to favorably perform color display.

Alternatively, the current driving device may comprise a transistor.With such constitution, it is possible to drive the EL light emittingelement by a programming current method.

Alternatively, the current driving device may comprise a current mirrorcircuit.

Alternatively, a plurality of pixels may be disposed in matrix; each ofthe pixels may be provided with the EL light emitting element and thecurrent driving device; each of columns or rows of the pixels may beprovided with the source signal line; the current driving devices of thecolumns or the rows may be connected selectively to the source signallines; each of the source signal lines may be provided with the signalcurrent source, the precharge voltage source, and the switching andconnecting unit; a plurality of gate lines for transmitting a gatesignal to select the current driving devices per row or column may beprovided; and a gate driver for outputting the gate signal to the gatelines may be provided.

An electronic display appliance according to the invention comprises: animage display device including the EL display apparatus according toclaim 1, wherein a plurality of pixels are disposed in matrix, each ofthe pixels is provided with the EL light emitting element and thecurrent driving device, each of columns or rows of the pixels isprovided with the source signal line, the current driving devices of thecolumns or the rows are connected selectively to the source signallines, each of the source signal lines is provided with the signalcurrent source, the precharge voltage source, and the switching andconnecting unit, a plurality of gate lines for transmitting a gatesignal to select the current driving devices per row or column areprovided, and a gate driver for outputting the gate signal to the gatelines is provided; a receiver; and a speaker. With such constitution, itis possible to realize an electronic display appliance of the EL displaymethod, which prevents the occurrence of flickering otherwise caused bythe reduction in frame frequency.

A driving circuit of the EL display apparatus according to the inventioncomprises: a plurality of unit current sources; a reference currentgeneration circuit for defining currents output from the unit currentsources; a plurality of current switching circuits disposed on outputends of the unit current sources; a current wiring of which one end isconnected to the current switching circuits via a first change overswitch and the other end is connected to source signal lines; and aprecharge voltage source which outputs a predetermined voltage and isconnected to the current wiring via a second change over switch, whereinthe current switching circuits are turned on and off depending on grayscale data of an image signal, and the first and the second change overswitches selectively connect either the current switching circuits orthe precharge voltage source to the source signal lines.

With such constitution, it is possible to realize a driving circuit ofthe EL display apparatus capable of preventing the occurrence offlickering due to the reduction in frame frequency.

The unit current sources may be connected to the current switches insuch a fashion that the number of the unit current sources to be alignedparallel and connected to each of the current switches is a multiple of2. With such constitution, it is possible to output source signals inaccordance with digital gray scale data.

The reference current generation circuit may have an operation amplifierso that the operation amplifier defines the currents output from theunit current sources.

These and other objects, characteristics, and advantages of theinvention will become apparent from the description of preferredembodiments which will hereinafter be described with reference toaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 2 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 3 is an explanatory diagram illustrating an operation of an ELdisplay panel according to the present invention.

FIG. 4 is an explanatory chart illustrating an operation of an ELdisplay panel according to the present invention.

FIG. 5 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 6 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 7 is an explanatory view illustrating a method of manufacturing anEL display panel according to the present invention.

FIG. 8 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 9 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 10 is a sectional view of an EL display panel according to thepresent invention.

FIG. 11 is a sectional view of an EL display panel according to thepresent invention.

FIG. 12 is an explanatory chart illustrating an EL display panelaccording to the present invention.

FIG. 13 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 14 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 15 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 16 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 17 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 18 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 19 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 20 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 21 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 22 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 23 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 24 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 25 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 26 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 27 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 28 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 29 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 30 is an explanatory view illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 31 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 32 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 33 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 34 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 35 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 36 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 37 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 38 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 39 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 40 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 41 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 42 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 43 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 44 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 45 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 46 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 47 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 48 is a diagram illustrating a configuration of an EL displayapparatus according to the present invention.

FIG. 49 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 50 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 51 is a diagram illustrating a pixel of an EL display panelaccording to the present invention.

FIG. 52 is an explanatory chart illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 53 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 54 is a diagram illustrating a pixel configuration of an EL displaypanel according to the present invention.

FIG. 55 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 56 is an explanatory diagram illustrating a method of driving an ELdisplay apparatus according to the present invention.

FIG. 57 is an explanatory view illustrating a mobile phone according tothe present invention.

FIG. 58 is an explanatory view illustrating a view finder according tothe present invention.

FIG. 59 is an explanatory view illustrating a digital video cameraaccording to the present invention.

FIG. 60 is an explanatory view illustrating a digital still cameraaccording to the present invention.

FIG. 61 is an explanatory view illustrating a television set (monitor)according to the present invention.

FIG. 62 is a diagram illustrating a pixel configuration of aconventional EL display panel.

FIG. 63 is a block diagram showing a driving circuit of the invention.

FIG. 64 is an illustration of the driving circuit of the invention.

FIG. 65 is an illustration of the driving circuit of the invention.

FIG. 66 is an illustration of the driving circuit of the invention.

FIG. 67 is an illustration of the driving circuit of the invention.

FIG. 68 is an illustration of the driving circuit of the invention.

FIG. 69 is an illustration of the driving circuit of the invention.

FIG. 70 is an illustration of the driving circuit of the invention.

FIG. 71 is a block diagram showing another driving circuit of theinvention.

FIG. 72 is an illustration of the driving circuit of the invention.

FIG. 73 is an illustration of the driving circuit of the invention.

FIG. 74 is an illustration of the driving circuit of the invention.

FIG. 75 is an illustration of the driving circuit of the invention.

FIG. 76 is an illustration of the driving circuit of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

For easy understanding and/or illustration, each of the drawings in thisdescription may have portions omitted and/or enlarged/reduced. Forexample, an encapsulating film 111 and the like are shown to be quitethick in the sectional view of a display panel at FIG. 11. On the otherhand, an encapsulating cover 85 is shown to be thin in FIG. 10. Thereare omitted portions. For example, a display panel or the like accordingto the present invention needs to have a phase film such as a circularlypolarizing plate for antireflection. However, such a phase film isomitted from the drawings used in this description. This holds true forother drawings. Like numerals, characters or the like designate partshaving identical or similar forms, materials, functions or operations.

It is to be noted that the details to be described with reference to thedrawings may be combined with other embodiments and the like. Forexample, a touch panel or the like may be added to a display panel shownin FIG. 8 to form an information display apparatus illustrated in anyone of FIGS. 19 and 59 to 61. Alternatively, a magnifying lens 582 maybe attached to the display panel to form a view finder (see FIG. 58) foruse in a video camera (see FIG. 59 and the like). Any one of the drivingmethods to be described with reference to FIGS. 4, 15, 18, 21 and 23 andlike figures is applicable to any one of display apparatus or displaypanels according to the present invention.

While driving transistors 11 and switching transistors 11 will bedescribed to be thin film transistors in this description, they are notlimited to thin film transistors. Each of the transistors 11 maycomprise a thin film diode (TFD), ring diode, or the like. Further, eachtransistor 11 is not limited to such a thin film device but may comprisea device formed on a silicon wafer. Of course, any one of FET, MOS-FET,MOS transistors and a bipolar transistor can serve the purpose. Theseare basically thin film transistors. It is needless to say that otherdevices such as a varistor, thyrister, ring diode, photodiode,phototransistor, and PLZT device can serve the purpose. That is, each ofthe switching devices 11 and driving devices 11 may comprise any one ofthe devices mentioned above.

As shown in FIG. 10, an organic EL display panel includes at least oneorganic functional layer (EL layer) 15 (15R, 15G and 15B) comprising anelectron transport layer, a luminescent layer, hole transport layer andthe like, and a metal electrode (reflective film) (cathode) 106, whichare stacked on a glass plate 71 (array substrate) formed with atransparent electrode 105 as a pixel electrode. The organic functionallayer (EL layer) 15 is caused to emit light by applying the anodeconsisting of the transparent electrode (pixel electrode) 105 and thecathode consisting of the metal electrode (reflective electrode) 106with a positive voltage and a negative voltage, respectively; statedotherwise, by applying direct current across the transparent electrode105 and the metal electrode 106.

A high current passes through wiring for feeding current to the anode orthe cathode (cathode wiring 86 or anode wiring 87 in FIG. 8). When thescreen size of an EL display apparatus is 40 inches for example, acurrent of about 100 A passes therethrough. Therefore, such wiring needsto have a sufficiently low value of resistance. To solve this problem,the present invention firstly forms thin film wiring to the anode or thelike (wiring for feeding EL devices with a luminescence-causingcurrent). The thin film wiring is then thickened with an electrolyticplating technique or an electroless plating technique.

Examples of metals for use in plating include chromium, nickel, gold,copper, and aluminum, or alloys, amalgams or laminated structuresthereof. As the need arises, the wiring is added with identical wiringor metal wiring comprising wiring and copper foil. Alternatively, thewiring is thickened to have decreased wiring resistance by screenprinting over the wiring with copper paste or the like to stack thepaste or the like thereon. The wiring may be reinforced by superpositionof additional wiring thereon using a bonding technique. As needsdictate, a grand pattern may be formed over the wiring to form acapacitor therebetween.

To feed the anode or cathode wiring with a high current, a power wirefor supply of a power having a low current and a high voltage is routedfrom current feeding means to a location in the vicinity of the anodewiring or the like and the power is converted into a power having a lowvoltage and a high current with a DCDC converter or the like beforebeing fed to the anode wiring or the like. That is, a high-voltage andlow-current wire is routed from the power source to a power-consumingtarget and the power fed therethrough is converted into a high-currentand low-voltage power at a location short of reaching thepower-consuming target. Examples of such converter means include a DCDCconverter, and a transformer.

Preferable materials for the metal electrode 106 include lithium,silver, aluminum, magnesium, indium and copper, or their respectivealloys or like materials having low work functions. Particularlypreferable is an Al-Li alloy for example. On the other hand, thetransparent electrode 105 may comprise a conductor material having ahigh work function, such as ITO, or gold or the like. If gold is used asthe electrode material, the resulting electrode is translucent. ITO maybe substituted with another material such as IZO. This holds true forother pixel electrodes 105.

In the vapor deposition of a thin film over the pixel electrode 105 orthe like, it is convenient to form organic EL film 15 in an argonatmosphere. By forming a carbon film having a thickness not less than 20nm and not more than 50 nm over ITO as the pixel electrode 105, anorganic EL film can be formed which exhibits improved interfacestability and satisfactory luminance and efficiency of luminescence. Theprocess for forming the EL film 15 is not limited to vapor deposition.It is needless to say that the EL film 15 may be formed using an ink jetprocess.

A desiccant 107 is placed in the space defined between the encapsulatingcover 85 and the array substrate 71. This is because the organic EL film15 is easily affected by humidity. The desiccant 107 absorbs moisturepermeating through sealant thereby preventing the organic EL film 15from deteriorating.

FIG. 10 shows an arrangement of encapsulation with cover 85 of glass.Encapsulation may be achieved using a film (which may be a thin film,i.e., encapsulating thin film) 111 as shown in FIG. 11. An example ofsuch an encapsulating film (encapsulating thin film) 111 is a filmformed by vapor deposition of DLC (diamond-like carbon) on a film foruse in electrolytic capacitors. This film has very poor waterpermeability (i.e. high moistureproofness) and hence is used as theencapsulating film 111. It is needless to say that an arrangement inwhich a DLC film or the like is vapor-deposited directly over theelectrode 106 can serve the purpose. Alternatively, the encapsulatingthin film may comprise a multi-layered film formed by stacking a resinthin film and a metal thin film on the other.

The thickness of the thin film is preferably established so that n·d isnot more than the dominant wavelength λ of light emitted from the ELdevice 15, wherein n represents the refractive index of the thin film(if plural thin films are stacked on each other, calculation is madewith their respective refractive indexes totalized (n·d is calculatedfor each thin film), and d represents the thickness of the thin film (ifplural thin films are stacked on each other, calculation is made withtheir respective refractive indexes totalized.) With this conditionbeing satisfied, the efficiency in taking light out of EL device 15 istwice or more as high as that of the case where encapsulation is madewith a glass substrate. An alloy, mixture or stack of aluminum andsilver may be formed as the encapsulating thin film.

Such encapsulation with encapsulating film 111 and without cover 85 asdescribed above is referred to as thin film encapsulation. In the thinfilm encapsulation to be applied to the case where light is taken outfrom the substrate 71 side, which is referred to as downward takeout(see FIG. 10 in which the arrow indicates the light takeout direction),an aluminum film to be used as the cathode is formed over the EL filmformed in advance. Subsequently, a resin layer to serve as a bufferlayer is formed over the aluminum film. Examples of materials for thebuffer layer include organic materials such as acrylic resin and epoxyresin. The thickness of the buffer layer is suitably not less than 1 μmand not more than 10 μm, more preferably not less than 2 μm and not morethan 6 μm. Further, encapsulating film 74 is formed over the bufferfilm. Without the buffer layer, the structure of the EL film wouldcollapse, causing streak-like defects to occur. As described above, theencapsulating film 111 comprises, for example, DLC (diamond-like carbon)or a layered structure for electrolytic capacitors (a multi-layeredstructure in which a dielectric thin film and an aluminum thin film areformed alternately by vapor deposition.)

In the thin film encapsulation to be applied to the case where light istaken out from the EL layer 15 side, which is referred to as upwardtakeout (see FIG. 10 in which the arrow indicates the light takeoutdirection), an Ag—Mg film to be used as the cathode (or the anode) isformed to a thickness not less than 20 angstroms and not more than 300angstroms over the EL film formed in advance. Subsequently, atransparent electrode comprising ITO or the like is formed over theAG-Mg film to lower the resistance, followed by the formation of a resinfilm as a buffer layer over the electrode film. Further, encapsulatingfilm 111 is formed over the buffer film.

A half of the amount of light emitted from the organic EL layer 15 isreflected by reflective film 106, passes through the array substrate 71,and is then emitted from the panel. However, undesired reflection occursdue to the reflective film 106 reflecting extraneous light, causing thedisplay contrast to lower. As the measures to avoid this inconvenience,a λ/4 plate 108 and a sheet polarizer (polarizing film) 109 are disposedat the array substrate 71. These are generally called a circularlypolarizing plate (circularly polarizing sheet).

If the pixels comprise a reflective electrode, light generated from theEL layer 15 is emitted upward. It is therefore needless to say that thephase plate 108 and the sheet polarizer 109 are disposed on thelight-emitting side in this case. Such reflective-type pixels can beobtained by forming pixel electrode 105 of aluminum, chromium, silver orthe like. If the surface of the pixel electrode 105 is provided withprojections (or projections and depressions), the interface with theorganic EL layer 15 is enlarged, which increases the light-emitting areaand improves the luminescence efficiency. It should be noted that whenit is possible to form a reflective film to serve as cathode 106 (oranode 105) on a transparent electrode or reduce the reflectance to 30%or lower, the circularly polarizing plate is unnecessary. This isbecause undesired reflection of extraneous light is reduced to a largeextent. Further, such an arrangement reduces interference of light andhence is desirable.

Preferably, each transistor 11 employs a LDD (lightly doped drain)structure. Though the organic EL device (which is variously abbreviatedas OEL, PEL, PLED, OLED or the like) 15 is exemplified as the EL devicein this description, it is needless to say that an inorganic EL deviceis applicable to the present invention without limitation to the organicEL device.

The active-matrix configuration used for the organic EL display panelhas to satisfy the following two conditions:

-   (1) the active-matrix configuration is capable of selecting a    specified pixel and giving the pixel required information; and-   (2) the active-matrix configuration is capable of passing a current    through each EL device throughout a one-frame period.

To satisfy these two conditions, the pixel configuration of theconventional organic EL device shown in FIG. 62 uses first transistor211 b as a switching transistor for pixel selection and secondtransistor 211 a as a driving transistor for feeding EL device (EL film)215 with current.

In causing this configuration to realize gray-scale display, the drivingtransistor 211 a needs to be applied a voltage corresponding to a levelof gray as a gate voltage. Accordingly, fluctuations of on-current inthe driving transistor 211 a are directly reflected in image display.

The on-current in a transistor formed of single crystal is extremelyinvariant, whereas a low-temperature polycrystalline transistor, whichis formed by the low temperature polysilicon technology which enablesthe formation of a transistor on an inexpensive glass substrate at 450°C. or lower, has a threshold voltage varying in the range from ±0.2 V to±0.5 V. For this reason, the on-current passing through the drivingtransistor 211 a fluctuates with variations in threshold voltage,resulting in display irregularities. Such irregularities occur due notonly to variations in threshold voltage but also to variations in themobility, gate insulator thickness or the like of the transistor. Also,the characteristics of the transistor 211 vary as the transistor 211deteriorates.

This phenomenon is possible to occur not only with the low temperaturepolysilicon technology but also with other technology including the hightemperature polysilicon technology using a processing temperature of450° C. or higher and the technology of forming a transistor using asemiconductor film resulting from solid phase (CGS) growth. As well, thephenomenon occurs with organic transistors and amorphous silicontransistors. Therefore, the present invention to be described below isdirected to configurations or methods capable of taking measuresdepending on those technologies. In this description, however,transistors of the type formed by the low temperature polysilicontechnology are described mainly.

With the method of gray scale display by writing with voltage as shownin FIG. 62, device characteristics need to be controlled precisely forproviding an invariant display. With the low temperature polysilicontransistor or the like presently available, however, the requirement ofcontrolling variations in device characteristics to within predeterminedranges cannot be satisfied.

In the pixel structure of the EL display apparatus according to thepresent invention, a unit pixel comprises four transistors 11 and an ELdevice, as specifically shown in FIG. 1. The pixel electrode is formedas overlapping the source signal lines. More specifically, source signallines 18 are insulated by the formation of an insulating film or aplanarizing film comprising an acrylic material over the source signallines 18, and then pixel electrode 105 is formed on the insulating film.Such a structure that a pixel electrode overlaps at least a part ofsource signal lines is called a high aperture (HA) structure. Thisstructure can be expected to reduce useless interference light andensure favorable luminescence.

When gate signal line (first scanning line) 17 a is rendered active(applied with on-voltage) by outputting of a gate signal thereto, thesource driver 14 feeds EL device 15 with a current having a valuerequired by EL device 15 through driving transistor 11 a and switchingtransistor 11 c associated with the EL device 15. By rendering gatesignal line 17 a active (applying the gate signal line with on-voltage)in a manner to shortcircuit the gate and the drain of the transistor 11a, the transistor 11 b is opened and, at the same time, the gate voltage(or the drain voltage) of the transistor 11 a is stored in capacitor(storage capacitor or additional capacitor) 19 connected between thegate and the source of the transistor 11 a (see FIG. 3( a).)

The capacitor 19 intermediate the source (S) and the gate (G) of thetransistor 11 a preferably has a capacitance of 0.2 pF or more. Astructure having capacitor 19 formed separately is exemplified asanother structure. That is, the structure has a storage capacitorcomprising a capacitor electrode layer, a gate insulator and gate metal.Such a separately-formed capacitor is preferable from the viewpoints ofpreventing a decrease in luminance due to leakage from the transistor 11c and stabilizing the display operation.

The capacitor (storage capacitor) 19 preferably has a capacitance notless than 0.2 pF and not more than 2 pF, particularly preferably notless than 0.4 pF and not more than 1.2 pF. The capacitance of thecapacitor 19 is determined in view of a pixel size. Assuming that Cs(pF) is the capacitance required for one pixel and Sp (square μm) is thearea occupied by one pixel (not the effective aperture ratio), Cs and Sppreferably satisfy 500/S≦Cs≦20000/S, more preferably1000/Sp≦Cs≦10000/Sp. Since the capacitance of the gate of the transistoris small enough, Q used here is the capacitance of the storage capacitor(capacitor) 19 alone.

Preferably, the capacitor 19 is formed substantially in a non-displayregion located intermediate adjacent pixels. Generally, in the formationof full color organic EL devices 15, misalignment of a mask causesmisregistration of organic EL layers to occur since the EL layers areformed using a vapor deposition process with a metal mask. Suchmisregistration might cause organic EL layers 15 (15R, 15G and 15B) forrespective colors to overlap each other. For this reason, adjacentpixels for respective colors have to be spaced 10μ or more by thenon-display region. This region does not contribute to luminescence.Therefore, the formation of storage capacitor 19 in this region is alsoeffective means for improving the effective aperture ratio.

Subsequently, gate signal line 17 a is rendered inactive (applied withoff-current) and gate signal line 17 b rendered active, so that thecurrent path is switched to the path including EL device 15 andtransistor 11 d connected to the first transistor 11 a and the EL device15, thereby causing the current stored in the aforementioned manner topass through the EL device 15 (see FIG. 3( b).)

This circuit has four transistors 11 in one pixel, the transistor 11 ahaving its gate connected to the source of the transistor 11 b. Thegates of the respective transistors 11 b and 11 c are connected to gatesignal line 17 a. The drain of the transistor 11 b is connected to thedrain of the transistor 11 c as well as the source of the transistor 11d. The source of the transistor 11 c is connected to source signal line18. The gate of the transistor 11 d is connected to gate signal line 17b, while the drain of the transistor 11 d connected to the anode of theEL device 15.

All the transistors shown in FIG. 1 are p-channel transistors. Thep-channel transistor is preferable because it has a high breakdownvoltage and is hard to deteriorate, though the p-channel transistorexhibits slightly lower mobility than the n-channel transistor. However,the present invention does not limit the transistors used in the ELdevice configuration to p-channel transistors. It is possible to formthe EL device configuration using the n-channel transistor exclusively.The EL device configuration may be formed using the n-channel transistorand the p-channel transistor both.

In FIG. 1, it is preferable that the transistors 11 c and 11 b have thesame polarity and are of the p-channel type while the transistors 11 aand 11 d are of the n-channel type. Generally, the p-channel transistoris characterized in the features including higher reliability and lessoccurrence of kink current than the n-channel transistor. Therefore, useis very effective of the p-channel transistor as the transistor 11 aassociated with EL device 15 which is designed to obtain a desiredintensity of luminescence by current control.

Most preferably, all the transistors forming a pixel as well asincorporated gate driver 12 are of the p-channel type. By thus formingthe array with exclusive use of p-channel transistors, the number ofmasks to be used is reduced to five, which can make the cost lower andthe yield higher.

For easy understanding of the present invention, description will bemade of the EL device configuration according to the present inventionwith reference to FIG. 3. The EL device configuration of the presentinvention is controlled with two timings. The first timing is timing forstoring a required current value. When transistors 11 b and 11 c areturned on at this timing, the equivalent circuit of the EL deviceconfiguration assumes the state shown in FIG. 3( a). Here, apredetermined current Iw is written through a signal line. By so doing,transistor 11 a is turned into a state where the gate and the drain areconnected to each other and current Iw passes through the transistor 11a and transistor 11 c. Accordingly, the voltage across the gate-sourceof transistor 11 a assumes a value such as to cause current Iw to pass.

The second timing is timing for closing transistors 11 b and 11 c andopening transistor 11 d. At this time, the equivalent circuit of the ELdevice configuration assumes the state shown in FIG. 3( b). The voltageacross the source-gate of transistor 11 a is held as it is. In this casetransistor 11 a operates within a saturation region at all times and,hence, the value of current assumes Iw constantly.

These operations cause the display apparatus to be driven as shown inFIG. 5. Reference character 51 a in FIG. 5( a) designates a pixel (row)of display screen 50 programmed with current at a certain time point(written pixel (row).) This pixel (row) 51 a is a non-lighting(non-display) pixel row as shown in FIG. 59( b). Other pixels (rows) aredisplay pixels (rows) 53. (That is, current is passing through ELdevices 15 of the display pixels (rows) 53 and the EL devices 15 areemitting light.)

In the case of the pixel configuration shown in FIG. 1, programmingcurrent Iw passes through source signal line 18 at the time ofcurrent-based programming. The current Iw passes through transistor 11 ato make voltage setting (programming) of the capacitor 19 so that avoltage such as to cause the current Iw to pass is held. At this timetransistor 11 d is open (in off-state).

In a period for allowing current to pass through EL device 15,transistors 11 c and 11 b are turned off while transistor 11 d turnedon, as shown in FIG. 3( b). Specifically, off-voltage (Vgh) is appliedto gate signal line 17 a to turn transistors 11 b and 11 c off. On theother hand, on-voltage (Vgl) is applied to gate signal line 17 d to turntransistor 11 d on.

The chart of such timing is shown in FIG. 4. In FIG. 4 and the like, aparenthesized additional numeral (for example, (1)) indicates a rownumber given to a pixel row. Specifically, gate signal line 17 a(1)indicates the gate signal line 17 a of pixel row (1). *H (“*” representsany character or numeral indicative of the number of a horizontalscanning line), which appears in the uppermost section of FIG. 4,represents a horizontal scanning period. Specifically, 1H represents thefirst horizontal scanning period. These matters are for easy descriptionand do not limit the number and the period of a one-H period, thesequence of pixel rows, and the like.

As seen from FIG. 4, in each pixel row selected (the period for whichthe pixel row is in the selected state is 1H), gate signal line 17 b isapplied with off-voltage, while gate signal line 17 a applied withon-voltage. In this period current does not pass through EL devices 15;that is, the EL devices 15 are in the non-lighting state. In each pixelrow unselected, on the other hand, gate signal line 17 a is applied withoff-voltage, while gate signal line 17 b applied with on-voltage. Inthis period current passes through EL devices 15; that is, the ELdevices 15 are in the lighting state.

The gate of transistor 11 b and that of transistor 11 c are connected tothe same gate signal line 17 a. However, they may be connected todifferent gate signal lines (the gate signal lines 17 a and 17 c in FIG.32). In this case, the number of gate signal lines associated with onepixel is three. (The configuration shown in FIG. 1 has two gate signallines for one pixel.) By individually controlling the on-off timing forthe gate of transistor 11 b and that for the gate of transistor 11 c,fluctuations in the value of current passing through EL devices 15 dueto variations in the characteristics of transistor 11 a can further bereduced.

If gate signal lines 17 a and 17 b formed into a common line andtransistors 11 c and 11 d are rendered different from each other inconductivity type (i.e., n-channel type and p-channel type), it ispossible to simplify the driving circuit and improve the effectiveaperture ratio of pixels.

With such a configuration, the writing path from a relevant signal linebecomes off at the operation timing according to present invention. Ifthe path allowing current to pass therethrough is branched when apredetermined value of current is to be written, the value of current isnot exactly stored in the capacitor located intermediate the source (S)and the gate (G) of transistor 11 a. Where transistors 11 c and 11 d arerendered different in conductivity type from each other, an operationbecomes possible such that transistor 11 d is necessarily turned onafter transistor 11 c has been turned off at timing at which a scanningline is switched to another if each other's threshold value iscontrolled.

Since the transistors require that each other's threshold value becontrolled accurately in this case, sufficient care is necessary in themanufacturing process. Though the above-described circuit is feasiblewith at least four transistors, a configuration having more than fourtransistors in which transistor 11 e is provided as cascade-connected asshown in FIG. 2 operates based on the same operating principle describedabove. Such a configuration with additional transistor 11 e can cause acurrent as exact as programmed through transistor 11 c to pass throughEL device 15.

Variations in the characteristics of transistor 11 a are correlated withthe size of the transistor 11 a. For reduction of such variations incharacteristics, the channel length of the first transistor 11 a ispreferably not less than 5 μm and not more than 100 μm, more preferablynot less than 10 μm and not more than 50 μm. This is because when thechannel length L is made longer, the grain boundary contained in thechannel increases, which is presumed to relax the electric field andhence lower the kink effect.

It is preferable that each of the transistors 11 forming a pixelcomprises a polysilicon transistor formed through the laserrecrystallization method (laser annealing) and the channels of all thetransistors extend in the same direction with respect to the laserirradiation direction. Further, it is preferable that the laser scansthe same portion twice or more to form a semiconductor film.

An object of the present invention is to propose a circuit configurationwhich prevents variations in transistor characteristics from affectingimage display. To attain this object, four or more transistors arenecessary. In determining a circuit constant from the characteristics ofthese transistors, it is difficult to determine a suitable circuitconstant unless the four transistors are made uniform incharacteristics. A transistor having a channel formed to extend in ahorizontal direction with respect to the longitudinal axis of laserirradiation is different in such transistor characteristics as thresholdvalue and mobility from a transistor having a channel formed to extendin a vertical direction with respect to the longitudinal axis of laserirradiation. The extent of variations in one case is the same as that inthe other. The transistor having the channel extending in the horizontaldirection and the transistor having the channel extending in thevertical direction are different from each other in a mean value ofmobility and a mean value of threshold. Thus, it is desirable that thechannel directions of all the transistors forming a pixel be the same.

Assuming that the capacitance of storage capacitor 19 is Cs and thevalue of off-current applied to the second transistor 11 b is Ioff, Csand Ioff preferably satisfy the formula: 3<Cs/Ioff<24.

More preferably, they satisfy the formula: 6<Cs/Ioff<18.

The variation in the value of current passing through EL devices can bereduced to 2% or less by adjusting off-current of transistor 11 b to 5pA or lower. This is because charge stored between the gate and thesource (opposite ends of the capacitor) cannot be maintained for aone-field period when voltage is not written. Therefore, with increasingstorage capacitance of the capacitor 19, allowable off-currentincreases. The variation in the value of current passing throughadjacent pixels can be reduced to 2% or less by satisfying theaforementioned formula.

It is preferable that each of the transistors forming the active-matrixconfiguration comprises a p-channel polysilicon thin film transistor andtransistor 11 b has a multi-gated structure having at least dual gate.Since transistor 11 b acts as a switch intermediate the source and thedrain of transistor 11 a, the highest possible on/off ratio is requiredof transistor 11 b. By employing such a multi-gated structure having atleast dual gate for the gate structure of transistor 11 b, a high on/offratio characteristic can be realized.

It is a general practice to form a semiconductor film constitutingtransistors 11 of pixels 16 through low temperature polysilicontechnology with laser annealing. Variations in laser annealingconditions result in variations in the characteristics of transistors11. However, if there is uniformity in the characteristics of respectivetransistors 11 in one pixel, a configuration adapted for current-basedprogramming as shown in FIG. 1 or the like is capable of operating sothat a predetermined current may pass through EL device 15. This featureis an advantage which a voltage-based programming configuration does nothave. The laser for use here is preferably an excimer laser.

In the present invention, the process used to form the semiconductorfilm is not limited to the laser annealing process but may be a thermalannealing process or a process based on solid phase (CGS) growth. It isneedless to say that the present invention can use not only the lowtemperature polysilicon technology but also the high temperaturepolysilicon technology.

In order to solve the problem described above, annealing is performed ina manner that a laser irradiation spot (laser irradiation range) 72extending parallel with source signal line 18 is irradiated with laserlight. Further, the laser irradiation spot 72 is moved so as to coincidewith one pixel column. Of course, there is no limitation to one pixelcolumn. One pixel unit 16 comprising R, G and B may be irradiated withlaser light (in this case three pixel columns are irradiated). It ispossible to irradiate plural pixels at a time. It is needless to saythat the laser irradiation range may be moved in an overlapping fashion.(Usually, moving laser irradiation range overlaps the preceding laserirradiation spot.)

Three pixels for R, G and B are formed to constitute a square shape.Accordingly, each of the pixels for R, G and B is vertically elongated.Thus, annealing with vertically elongated laser irradiation spot 72makes it possible to avoid the occurrence of variations in thecharacteristics of transistors 11 in one pixel. Further, the transistors11 connected to one source signal line 18 can be rendered uniform incharacteristics (mobility, Vt, S value and the like.) (That is, thetransistors 11 connected to one source line 18 can be made substantiallyto agree to each other in characteristics, though there may be a casewhere the transistors 11 connected to one source signal line 18 aredifferent in characteristics from those connected to an adjacent signalline 18.)

Generally, the length of laser irradiation spot 72 is a fixed value, forexample 10 inches. Since laser irradiation spot 72 moves, the panelneeds to be positioned so that one laser irradiation spot 72 can movewithin a range allowing laser irradiation spot 72 to move therein. (Thatis, the panel needs to be positioned so as to prevent laser irradiationspots 72 from overlapping each other in a central portion of displayregion 50 of the panel.)

In the arrangement shown in FIG. 7, three panels are formed as arrangedvertically within a range corresponding to the length of laserirradiation spot 72. An annealing apparatus for irradiation of laserirradiation spot 72 recognizes positioning markers 73 a and 73 bprovided on glass substrate 74 (automatic positioning based on patternrecognition) and moves laser irradiation spot 72. The positioningmarkers 73 are recognized by means of a pattern recognition device. Theannealing apparatus recognizes the positioning markers 73 to find theposition of a pixel column. (That is, the apparatus makes laserirradiation range 72 parallel with source signal line 18.) Sequentialannealing is performed through irradiation of laser irradiation spot 72positioned coinciding with the position of each pixel column.

Use of the laser annealing method (of the type adapted for irradiationof a linear laser spot extending parallel with source signal line 18)described with reference to FIG. 7 is preferable particularly inmanufacturing an organic EL display panel of the current-basedprogramming type. This is because transistors 11 arranged parallel witha source signal line are uniform in characteristics. (That is, thecharacteristics of one pixel transistor are approximate to those of avertically adjacent pixel transistor.) For this reason fluctuations inthe voltage level of a source signal line which occur in current-baseddriving are small and, hence, insufficient writing with current is notlikely to occur.

In the case of white raster display for example, a current to be passedthrough transistor 11 a of one pixel is substantially equal to a currentto be passed through transistor 11 a of an adjacent pixel and,therefore, the amplitude of a current outputted from source driver 14varies little. If transistors 11 a in FIG. 1 are uniform incharacteristics and the values of currents for programming pixels of apixel column are equal to each other, fluctuations in the potential ofsource signal line 18 do not occur. Accordingly, if the transistors 11 aconnected to one source signal line 18 are substantially uniform incharacteristics, fluctuations in the potential of the source signal line18 are small. This also holds true for other pixel configurations of thecurrent-based programming type as shown in FIG. 38 and the like. (Thismeans that use of the manufacturing method illustrated in FIG. 7 ispreferable.)

Uniform image display can also be realized by a configuration of thetype adapted for writing to plural pixel rows at a time to be describedwith reference to FIG. 27 or 30 or the like. This is mainly becausedisplay irregularities due to variations in transistor characteristicsare not likely to occur. Since the configuration shown in FIG. 27 or thelike selects plural pixel rows at a time, driver circuit 14 canaccommodate variations in the characteristics of transistors arrangedvertically if the transistors of adjacent pixel rows are uniform.

Though the source driver 14 is formed as comprising an IC chip as shownin FIG. 7, the formation of source driver 14 is not limited thereto. Itis needless to say that source driver 14 may be formed together withpixels 16 in the same process.

In the present invention, particularly, the threshold voltage Vth2 oftransistor 11 b is established so as not to be lower than the thresholdvoltage Vth1 of transistor 11 a associated with transistor 11 b in onepixel. For example, the gate length L2 of transistor 11 b is made longerthan the gate length L1 of transistor 11 a so that Vth2 may not becomelower than Vth1 even when the process parameters of these thin filmtransistors vary. By so doing, faint leakage current can be inhibited tooccur.

The above-described features are also applicable to the current mirrorpixel configuration shown in FIG. 38. The configuration shown in FIG. 38comprises driving transistor 11 a allowing signal current to passtherethrough, driving transistor 11 b for controlling driving current tobe passed through a light-emitting device comprising EL device 15 or thelike, take-in transistor 11 c for connecting or disconnecting the pixelcircuit to or from a data line (data) by control over gate signal line17 a 1, switching transistor 11 d for shortcircuiting the gate and thedrain of transistor 11 a during a writing period by control over gatesignal line 17 a 2, storage capacitor 19 for holding a voltage acrossthe gate and the source of transistor 11 a even after completion ofwriting of the voltage, and EL device 15 as a light-emitting device.

Though transistors 11 c and 11 d are n-channel transistors while othertransistors are p-channel transistors in FIG. 38, this feature is a mereexample and the configuration need not necessarily have this feature.Though the storage capacitor 19 has one terminal connected to the gateof transistor 11 a and the other terminal connected to Vdd (power supplypotential), the storage capacitor 19 may be connected to any fixedpotential instead of Vdd. The cathode (negative electrode) of EL device15 is connected to the ground potential.

Description will be made of an EL display panel and an EL displayapparatus according to the present invention. FIG. 6 is an explanatorydiagram mainly illustrating the circuit of the EL display apparatus.Pixels 16 are arranged or formed in a matrix pattern. Each pixel 16 isconnected to source driver 14 adapted to output a current forcurrent-based programming of each pixel 16. The source driver 14 has anoutputting section formed with current mirror circuits corresponding tothe number of bits of an image signal as gray scale data, as will bedescribed later. For example, if there are 64 gray-levels, each sourcesignal line is formed with 63 current mirror circuits. The source driver14 is configured to be capable of applying a desired current to sourcesignal line 18 by selecting a current mirror circuits count.

The minimum output current of one current mirror circuit is set to benot more than 10 nA and not less than 50 nA. It is particularlypreferable to set the minimum output current of one current mirrorcircuit to be not more than 15 nA and not less than 35 nA. This isbecause such setting can ensure correct functioning of the transistorsforming the current mirror circuits in the source driver 14.

The source driver 14 incorporates a precharge or discharge circuit forforcibly charging or discharging source signal line 18. The precharge ordischarge circuit for forcibly charging or discharging source signalline 18 is preferably configured to be capable of setting output voltage(current) values for respective of R, G and B independently. This isbecause EL devices 15 for R, G and B have different threshold values.

Organic EL devices are known to have high temperature dependence. Inorder to control variations in luminance intensity due to suchtemperature dependence, the current mirror circuits are provided with anonlinear device, such as thermistor or posister, for varying the outputcurrent. A reference current is generated in an analog fashion byadjusting variations due to the temperature dependence by means of thethermistor or the like.

In the present invention, source driver 14 comprises a semiconductorchip and is connected to terminals of source signal lines 18 onsubstrate 71 by the Chip On Glass (COG) technology. Metal wires ofchromium, aluminum, silver or the like are used for wiring of signallines including source signal lines 18. This is because such a wireoffers a low resistance with a small wiring width. In the case where thepixels are of the reflection type, it is preferable that such wiring ismade of the same material as the reflective film of the pixels andformed at the same time with the formation of the reflective film. By sodoing, the process can be simplified.

The technology for use in mounting source driver 14 is not limited tothe COG technology. It is possible that the source driver 14 is mountedby the Chip On Film (COF) technology and connected to signal lines ofthe display panel. A drive IC may comprise three chips, with a powersupply IC 82 being formed separately.

On the other hand, the gate driver 12 is formed by the low temperaturepolysilicon technology. This means that the gate driver 12 is formedalong with the transistors of the pixels by the same process. This isbecause the gate driver 12 has a simple internal structure and a lowworking frequency as compared to the source driver 14. Therefore, thegate driver 12 can be formed easily even by the low temperaturepolysilicon technology, which leads to the frame made narrower. Ofcourse, it is needless to say that the gate driver 12 may comprise asilicon chip and may be mounted on the substrate 71 by utilizing the COGtechnology. The gate driver, switching devices including a pixeltransistor, and like components may be formed by the high temperaturepolysilicon technology, or they may be formed using an organic material(organic transistor).

The gate driver 12 incorporates a shift register circuit 61 a for gatesignal line 17 a, and a shift register circuit 61 b for gate signal line17 b. Each shift register 61 is controlled using clock signals ofpositive and negative phases (CLKxP and CLKxN) and start pulse (STx).Preferably, there are additionally used an enable signal (ENABL) forcontrolling outputting/non-outputting from gate signal lines and anup-down (UPDOWN) signal for reversing the shifting direction up anddown. It is also preferable to provide an output terminal or the likefor checking whether the start pulse has been shifted by the shiftregister and outputted therefrom. The timing for shifting by the shiftregister is controlled using a control signal from control IC 81. Thegate driver 12 further incorporates a level shifting circuit forshifting an extraneous data level, and an inspection circuit.

Since the shift register circuit 61 has a low buffer capacity, the shiftregister circuit 61 cannot directly drive gate signal lines 17. For thisreason, at least two inverter circuits 62 are formed between the outputof the shift register 61 and an associated output gate 63 adapted todrive gate signal line 17.

Similarly, in the case where the source driver 14 is formed directly onthe substrate 71 by such polysilicon technology as the low temperaturepolysilicon technology, plural inverter circuits are formed between ananalog switch gate such as a transfer gate for driving source signalline 18 and a shift register of the source driver 14. The source driverand the gate driver share the following feature (i.e., the featurerelated to an inverter circuit provided between the output of a shiftregister and an outputting section (including an output gate or atransfer gate)) adapted to drive signal lines.

Though an output of the source driver 14 is shown to connect directly tosource signal line 18 in FIG. 6 for example, actually the output of theshift register of the source driver is connected to multiple invertercircuits, the outputs of which are connected to analog switch gates suchas transfer gates.

Each inverter circuit 62 comprises a p-channel MOS transistor and ann-channel MOS transistor. As described above, an output terminal ofshift register 61 of the gate driver 12 is connected to multipleinverter circuits 62 and the output of the final inverter circuit isconnected to associated output gate circuit 63. Each inverter circuit 62may comprise transistors of p-channel type only. In this case, invertercircuit 62 may serve as a mere gate circuit but not as an inverter.

FIG. 8 is a diagram illustrating an arrangement for supply of signalsand voltage in the display apparatus or the configuration of the displayapparatus according to the present invention. Signals from control IC 81are fed to source driver 14 a (power supply wiring, data wiring or thelike) through flexible board 84.

In FIG. 8, control signals for gate driver 12 are generated at controlIC 81, level-shifted at source driver 14 and then applied to gate driver12. Since the driving voltage of source driver 14 ranges from 4 to 8(V), a control signal having an amplitude of 3.3 (V) can be convertedinto a signal having an amplitude of 5 (V), which can be received bygate driver 12.

Source driver 14 is preferably provided therein with image memory. Theimage memory may store image data previously subjected to an errordiffusion process or a dither process. Such an error diffusion processor dither process can convert 260,000-color display data into, forexample, 4096-color display data, thereby contributing to a reduction inthe capacity of the image memory. The error diffusion process or thelike can be achieved with error diffusion controller 81. Image data maybe subjected to the dither process and then further subjected to theerror diffusion process. The matter described above holds true for areverse error diffusion process.

Though the component 14 in FIG. 8 or the like is referred to as thesource driver, the component 14 may incorporate not only a mere drivercircuit but also a power supply circuit, buffer circuit (including sucha circuit as a shift register), data converter circuit, latch circuit,command decoder, shift circuit, address translator circuit, image memoryor the like. It is needless to say that a three-side-free arrangement(structure) and a driving method, which will be described with referenceto FIG. 9 and the like, are applicable to the configuration describedwith reference to FIG. 8.

For the display panel to be used in an information display apparatussuch as a mobile phone, it is preferable that source driver (circuit) 14and gate driver (circuit) 12 are mounted (formed) on one side of thedisplay panel. (It should be noted that an arrangement such that driverICs (circuits) are mounted (formed) on one side of a panel is referredto as a three-side-free arrangement (structure). It has been aconventional practice to mount gate driver 12 and source driver 14 onX-side and Y-side, respectively, of a display region.) Thethree-side-free arrangement allows the center line of screen 50 tocoincide with the center line of the display apparatus easily and makesthe mounting of driver ICs easy. The gate driver may be formed in athree-side-free arrangement by the high temperature or low temperaturepolysilicon technology. (That is, at least one of source driver 14 andgate driver 12 shown in FIG. 9 is formed directly on substrate 71 by thepolysilicon technology.)

The term “three-side-free arrangement” is meant to include not only anarrangement having ICs mounted or formed directly on substrate 71 butalso an arrangement in which a film attached with source driver(circuit) 14, gate driver (circuit) 12 and the like (by TCP or TABtechnology) is bonded to one side (or essentially one side) of substrate71. That is, the term “three-side-free arrangement” is meant to includeany arrangement or disposition having two sides on which any IC is notmounted or fitted as well as all arrangements similar thereto.

When gate driver 12 is disposed beside source driver 14 as shown in FIG.9, gate signal lines 17 need to be arranged along side C.

The portion indicated by thick solid line in FIG. 9 and the like is aportion in which gate signal lines are formed side by side. Accordingly,the portion designated by reference character b (lower portion in thefigure) is formed with parallel gate signal lines 17 in the numbershown, while the portion designated by reference character a (an upperportion in the figure) is formed with one gate signal line 17.

The pitch at which gate signal lines 17 are formed on C side is not lessthan 5 μm and not more than 12 μm. If the pitch is less than 5 μm, noiseoccurs at an adjacent gate signal line by the influence of parasiticcapacity. According to an experiment, the influence of parasiticcapacity becomes significant when the pitch is 7 μm or less. When thepitch further decreases to a value less than 5 μm, image noise such asbeat noise occurs vigorously on the display screen. Particularly, noiseoccurs differently between the right-hand side and the left-hand side ofthe screen and it is difficult to reduce such image noise as beat noise.On the other hand, if the pitch exceeds 12 μm, the frame width D of thedisplay panel becomes so large that the display panel cannot be put topractical use.

The aforementioned image noise can be reduced by providing a groundpattern (which is a conductive pattern set to have a fixed voltage or astabilized potential as a whole) as a layer underlying or overlying theportion formed with gate signal lines 17. Alternatively, aseparately-formed shielding plate or foil (which is a conductive patternset to have a fixed voltage or a stabilized potential as a whole) shouldbe placed over gate signal lines 17.

Though the gate signal lines 17 formed on side C in FIG. 9 may comprisean ITO electrode each, each of them preferably comprise a stack of ITOfilm and metal thin film so as to have decreased resistance.Alternatively, each gate signal line preferably comprises a metal film.In stacking metal thin film on ITO, a titanium film is formed over ITOand then a thin film of aluminum or of alloy comprising aluminum andmolybdenum is formed over the titanium film. Alternatively, a chromiumfilm is formed over ITO. In the case where each gate signal linecomprises metal film, the metal film comprises an aluminum thin film ora chromium thin film. The matters described above hold true for otherembodiments of the present invention.

There is no limitation to the arrangement shown in FIG. 9 or the like inwhich gate signal lines 19 are disposed (or formed) on one side ofdisplay region 50. Gate signal lines 19 may be disposed (or formed) onopposite sides of display region 50. For example, it is possible thatgate signal lines 17 a are disposed (or formed) on the right-hand sideof display region 50 while gate signal lines 17 b disposed (or formed)on the left-hand side of display region 50. The matter thus describedhold true for other embodiments.

Source driver 14 and gate driver 12 may be formed into a single chip.With such a single chip, it is sufficient to mount a single IC chip onthe display panel. Accordingly, the mounting cost can be reduced. Inaddition, different voltages to be used in the single chip driver IC canbe generated at a time.

There is no limitation to the above-described feature that source driver14 and gate driver 12 are each formed from a semiconductor wafer such assilicon and then mounted on the display panel. It is needless to saythat they may be formed directly on display panel 82 by the lowtemperature polysilicon technology or the high temperature polysilicontechnology.

In the configuration shown in FIG. 1 or the like, EL device 15 isconnected to Vdd potential through transistor 11 a. Such aconfiguration, however, involves a problem of different driving voltagesto be applied to organic EL devices for developing respective colors.For example, when a current of 0.01 (A) is allowed to pass per unit cm²,the terminal voltage of EL device for blue (B) assumes 5 (V) while thatof each of EL devices green (G) and red (R) assumes 9 (V). That is, Gand R are different from B in terminal voltage. Therefore, B isdifferent from G and R in the source-drain voltage (SD voltage) oftransistor 11 a to be held. For this reason, the transistors associatedwith respective color EL devices have different off-leak currents due todifferent source-drain voltages (SD voltages). When such off-leakcurrents occur with a difference in off-leak characteristic between ELdevices for respective colors, a complicated display state results whereflicker occurs with the colors being out of balance and the gammacharacteristic deviates in accordance with the correlation with thecolor of emitted light.

To deal with this problem, an arrangement is employed such that thepotential at the cathode of at least one of R, G and B devices is madedifferent from that at the cathode of each of the other devices.Alternatively, another arrangement may be employed such that the Vddpotential of at least one of R, G and B devices is made different fromthat of each of the other devices.

It is needless to say that terminal voltages of EL devices for R, G andB are preferably made as equal to each other as possible. Materials andstructures needs to be selected so that the terminal voltages of R, Gand B devices assume respective values not higher than 10 (V) oncondition that the devices each exhibits a white peak luminance and thecolor temperatures of the respective devices are in the range not lowerthan 7000 K and not higher than 12,000 K. Further, the differencebetween the maximum terminal voltage and the minimum terminal voltage ofthe EL devices for R, G and B need be not more than 2.5 (V), preferablynot more than 1.5 (V). While the foregoing embodiment uses the colors ofR, G and B, there is no limitation to these colors. This will bedescribed later.

While the pixels are adapted to develop the three primary colors, namelyR, G and B, they may be adapted to develop three colors, namely cyan,yellow and magenta. It is possible to use two colors, namely B andyellow. Of course, it is possible to use a monochromatic color. It ispossible to use six colors, namely R, G, B, cyan, yellow and magenta. Itis also possible to use five colors, namely R, G and B, cyan andmagenta. These colors offer widened color reproducible ranges of naturalcolors and hence are capable of realizing favorable display. Anotherpossible combination of colors includes four colors, namely R, G, B andwhite. Yet another possible combination of colors includes seven colors,namely R, G, B, cyan, yellow, magenta, black and white. It is possiblethat white light emitting pixels are formed (or made) throughout displayregion 50 and R, G and B color filters are provided on the pixels torealize a three-primary-color display. In this case it is sufficient tostack light-emitting materials for respective colors on EL layers.Alternatively, each pixel is dividedly painted with B and yellow forexample. As described above, the El display apparatus according to thepresent invention is not limited to color display based on the R, G andB three primary colors.

Three major methods can be used in causing an organic EL display panelto realize color display, and the color conversion method is one ofthem. According to this method, it is sufficient to form a singleluminescent layer for blue and the other colors, namely green and red,required for full color display are produced by color conversion fromblue light. Accordingly, there is no need to provide layers painted intoR, G and B separately. This method has an advantage that there is noneed to provide a set of organic EL materials for respective of R, G andB. The color conversion method is free of a decrease in productionyield, which is essential to the separately painting method. Eithermethod is applicable to the EL display panel and the like according tothe present invention.

In addition to the pixels for the three primary colors,white-light-emitting pixels may be formed. Such a white-light-emittingpixel can be realized by stacking light-emitting structures for R, G andB on each other. A set of pixels comprises pixels for the R, G and Bthree primary colors and a white-light-emitting pixel 16W. The formationof such a white-light-emitting pixel makes it easy to develop a whitelight peak luminance. Thus, brilliant image display can be realized.

In forming a set of pixels for the R, G and B three primary colors orlike colors, the pixels for the respective colors are preferably made tohave respective pixel electrodes having different areas. Of course, thepixel electrodes may have equal areas if the emission efficiencies ofthe respective colors are well-balanced and the color purities of therespective colors are also well-balanced. If one or plural colors areill-balanced, it is preferable to adjust the light-emitting surfaceareas of the respective pixel electrodes. The light-emitting surfaceareas of the pixel electrodes for the respective colors should bedetermined based on their current densities. Specifically, on conditionthat white balance is adjusted in a state where the color temperaturesare within the range not lower than 7000 K (Kelvin) and not higher than12,000 K, the difference in current density between the pixel electrodesfor the respective colors is adjusted to within ±30%, preferably ±15%.If the current density of the pixel electrode for one color is 100 A/m²for example, the current density of the pixel electrode for any one ofthe three primary colors is made to assume a value not less than 70 A/m²and not more than 130 A/m², more preferably not less than 85 A/m² andnot more than 115 A/m².

Organic EL device 15 is a self-luminescent device. When light ofluminescence becomes incident on a transistor serving as a switchingdevice, a photoconductor phenomenon occurs. The photoconductorphenomenon is a phenomenon that leakage at a switching device, such as atransistor, in an off-state (off-leak) increases due to opticalexcitation.

To deal with this problem, the present invention forms a light-shieldingfilm underlying gate driver 12 (source driver 14 in some cases) andpixel transistors 11. The light-shielding film comprises a metal thinfilm such as chromium and has a thickness not less than 50 nm and notmore than 150 nm. If the film thickness is too small, the film has apoor light-shielding effect. On the other hand, if the film thickness istoo large, unevenness occurs, which makes the patterning of overlyingtransistors 11 a difficult.

A planarization film having a thickness not less than 20 nm and not morethan 100 nm, which comprises an inorganic material, is formed over thelight-shielding film. One electrode of storage capacitor 19 may beformed using the layer of this light-shielding film. In this case theplanarization film is preferably made as thin as possible so that thestorage capacitor has a larger capacitance. Alternatively, it ispossible that the light-shielding film is formed from aluminum and asilicon oxide film is formed over the surface of the light-shieldingfilm by utilizing the anodic oxidation technique for use as a dielectricfilm of storage capacitor 19. On the planarization film are formed pixelelectrodes of a high aperture (HA) structure.

The driver circuit 12 and the like should inhibit penetration of lightnot only from the reverse side but also from the obverse side. This isbecause malfunction of such a circuit is caused by the influence of thephotoconductor phenomenon. For this reason, in the present invention,when the cathode comprises a metal film, the drivers 12 and the like areformed with such a cathode electrode covering the surface thereof toserve as the light-shielding film.

However, the formation of such a cathode over the drivers 12 possiblycauses a malfunction of the drivers due to an electric field producedfrom the cathode or an electric contact between the cathode and thedriver circuit. To deal with this problem, the present invention formsat least one organic EL film layer, preferably a plurality of organic ELfilm layers over the driver circuits 12 and the like at the same timewith the formation of the organic EL film over pixel electrodes.

Since such an organic EL film is basically an insulator, the formationof the organic EL film over the drivers isolates the drivers from thecathode, thus overcoming the aforementioned problem.

When shortcircuiting occurs between terminals of one or more transistors11 or between a signal line and a transistor 11, EL device 15 associatedtherewith lights constantly and such a pixel may become a luminescentspot. Since this luminescent spot is visually prominent, the luminescentspot needs to be turned into a black spot (or turned into thenon-lighting state.) The pixel 16 constituting such a luminescent spotis detected and then the capacitor 19 of the pixel 16 is irradiated withlaser light so that the terminals thereof are shortcircuited. By sodoing, the capacitor 19 becomes incapable of holding charge and, hence,the transistor 11 a cannot allow current to pass therethrough any more.

It is desirable that the cathode film situated in a region to beirradiated with laser light be removed in advance in order to prevent aterminal electrode of the capacitor 19 from shortcircuiting with thecathode film.

A defect of transistor 11 of pixel 16 affects the driver circuit 14 orthe like. For example, when a source-drain (SD) shortcircuit 562 occursat driving transistor 11 a as shown in FIG. 56, the source driver 14 isapplied with Vdd voltage of the panel. For this reason, the supplyvoltage of the source driver 14 is preferably set equal to or higherthan the supply voltage Vdd of the panel. It is preferable to employ anarrangement capable of controlling the reference current to be used inthe source driver 14 by means of an electron volume 561.

When SD shorcircuit 562 occurs at transistor 11 a, an excessive currentpasses through EL device 15. This causes the EL device 15 to lightconstantly (to become a luminescent spot). Such a luminescent spot isvisually prominent as a defect. In FIG. 56 for example, when asource-drain (SD) shortcircuit occurs at transistor 11 a, current fromthe Vdd voltage keeps on passing through the EL device 15 (while thetransistor 11 d is on.) Accordingly, the EL device 15 becomes aluminescent spot.

Further, such a SD shorcircuit at the transistor 11 a causes the Vddvoltage to be applied to source signal line 14, hence, to the sourcedriver 14 while the transistor 11 c is on. If the supply voltage of thesource driver 14 is lower than Vdd, the source driver 14 might be brokendown due to a voltage exceeding the withstand voltage. For this reason,the supply voltage of the source driver 14 is preferably set equal to orhigher than the Vdd voltage (which is the higher voltage applied to thepanel.)

The SD shortcircuit or a like defect at transistor 11 a may result inthe breakdown of the source driver of the panel as well as a spotdefect. A luminescent spot, which is visually prominent, makes the panelfaulty. For this reason, it is necessary to turn such a luminescent spotinto a black defect by cutting off the wiring interconnecting transistor11 a and EL device 15. Optical means such as laser light may be used tocut off the wiring.

Though wiring is cut off in the above embodiment, the means for changinga luminescent spot into a black display spot is not limited thereto. Ascan be understood from FIG. 1 for example, a modification may be made sothat the supply voltage Vdd for transistor 11 a is constantly applied tothe gate (G) terminal of the transistor 11 a. For example, if theopposite terminals of the capacitor 19 are shortcircuited, the Vddvoltage is applied to the gate (G) terminal of transistor 11 a.Accordingly, the transistor 11 a is kept in complete off-state and hencedoes not allow current to pass through the EL device 15 any more. Thiscan be easily realized through laser irradiation of capacitor 19, whichcan shortcircuit the capacitor electrodes.

Further, since the Vdd wiring actually underlies the pixel electrode,the display condition of the pixel can be controlled (or modified)through irradiation of the Vdd wiring and the pixel electrode with laserlight.

Additionally, turning a luminescent spot into a black defect can also berealized by making open the channel between the source and the drain ofthe transistor 11 a. Briefly, the transistor 11 a is irradiated withlaser light to make the channel thereof open. Similarly, the channel ofthe transistor 11 d may be opened. When the channel of the transistor 11b is opened, the associated pixel 16 cannot be selected and hencebecomes a black display.

In order to turn pixel 16 into a black display, the EL device 15 may bedeteriorated. For example, laser light is applied to the EL layer 15 todeteriorate the EL layer physically or chemically, thereby making the ELlayer 15 incapable of luminescence (constant black display.) Irradiationwith laser light can heat the EL layer 15 thereby deteriorating iteasily. Use of an excimer laser can cause a chemical change of the ELlayer 15 to take place easily.

While the pixel configuration shown in FIG. 1 is exemplified in theabove-described embodiment, the present invention is not limitedthereto. It is needless to say that the art of making wiring orelectrodes open or shortcircuited by the use of laser light isapplicable to other current-driven pixel configurations such as acurrent mirror circuit configuration and voltage-driven pixelconfigurations as shown in FIG. 62 or 51 or the like.

A method of driving the pixel configuration shown in FIG. 1 will bedescribed below. As shown in FIG. 1, gate signal line 17 a assumes aconducting state during a row selecting period, while gate signal line17 b assumes a conducting state during an unselecting period. (Here,application of a low-level voltage causes gate signal line 17 to assumethe conducting state since the transistors 11 in FIG. 1 are p-channeltransistors.)

Parasitic capacitance (not shown) is present in source signal line 18.Such parasitic capacity is produced due to a capacitance at each of theintersections of source signal line 18 and gate signal lines 17, achannel capacitance at each of transistors 11 b and 11 c, or the like.

Time t required for the value of current at source signal line 18 tovary is found from the equation: t=C·V/I, where C represents the valueof parasitic capacity, V represents a voltage applied to source signalline 18 and I represents a current passing through source signal line18. Accordingly, the time t required for the value of current to varycan be shortened to nearly 1/10 by increasing current to a 10-foldvalue. The equation also indicates that even when the parasitic capacityin source signal line 18 increases to a 10-fold value, the value ofcurrent can be varied to a predetermined value. Therefore, increasingthe value of current is effective in writing a predetermined currentvalue within a short horizontal scanning period.

In order to charge/discharge the parasitic capacity of source signalline 18, a current having value I satisfying the formula: I>(C·V)/tshould be passed through source signal line 18.

If the input current is increased 10 times, the output current is alsoincreased 10 times. In this case the luminance of the EL device is alsoraised 10 times, which means that a predetermined luminance cannot beobtained. In this respect, the present invention realizes thepredetermined luminance by providing settings such that the conductingperiod of transistor 17 d in FIG. 1 is set to 1/10 of the conventionalconducting period and the light-emitting period of EL device 15 set to1/10 of the conventional light-emitting period.

That is, in order to program transistor 11 a of pixel 16 with apredetermined current value after sufficient charge/discharge of theparasitic capacity of source signal line 18, source driver 14 needs tooutput a relatively high current. However, when such a high current ispassed through source signal line 18, the pixel is programmed with thevalue of this current undesirably, with the result that the EL device 15is fed with a higher current than the predetermined current. Forexample, if programming is made with a 10-fold current, naturally a10-fold current passes through EL device 15, thus causing the EL device15 to emit light at a 10-fold luminance. To obtain the predeterminedluminance of emission, the time period for which the EL device 15 is fedwith the current should be shortened to 1/10. Such a driving method iscapable of sufficiently charging/discharging the parasitic capacity ofsource signal line 18 and obtaining the predetermined luminance ofemission.

The above-described feature that a 10-fold current value is written totransistor 11 a of a pixel (more exactly, the terminal voltage ofcapacitor 19 is set to a predetermined value) and the on-time of ELdevice 15 is shortened to 1/10, is an mere example. In some cases it ispossible that a 10-fold current value is written to transistor 11 a of apixel and the on-time of EL device 15 is shortened to ⅕. Alternatively,as the case may be, it is possible that a 10-fold current value iswritten to transistor 11 a of a pixel and the on-time of EL device 15 isshortened to ½.

The present invention is characterized by a driving method in which acurrent to be written to a pixel is set to have a value different fromthe predetermined value while EL device 15 is fed with a currentintermittently. For easy explanation, the driving method is hereindescribed as having a feature that a current N times as high as thepredetermined current is written to transistor 11 of a pixel while theon-time of EL device 15 is set 1/N times the predetermined time period.However, the present invention is not limited to this feature. It isneedless to say that it is possible that an N1-fold current is writtento transistor 11 of a pixel while the on-time of EL device 15 is 1/N2times as large as the predetermined time period, (where N1 and N2 aredifferent from each other.)

The “predetermined current”, as used herein, means a current required torealize a gray scale display corresponding to an image signal. Thepredetermined current has a current value varying depending on thespecifications of the EL display apparatus. For example, the currentvalue ranges from about 0.25 μA to about 0.75 A when a luminance of 150nt is to be realized. Therefore, if N=4, a current value of from about 1μA to about 3 μA is to be written to transistor 11. Similarly, if N=8,the current value to be written ranges from about 2 μA to about 6 μA. IfN=2, the current value to be written ranges from about 0.5 μA to about1.5 μA.

The intervals at which the intermittent passage of current is performedare not limited to equal intervals. For example, random intervals arepossible (provided the display period or the non-display period, as awhole, has a predetermined value (fixed ratio).) The intervals maydiffer depending on R, G and B. That is, each of R, G and B displayperiods or non-display periods should be adjusted to a predeterminedvalue (fixed ratio) so as to optimize the white balance.

For easy explanation, the on-time is described to be 1/N of 1F (onefield or one frame period), 1F being used as a reference. However, atime period required for selection of one pixel row and programming witha current value (which is usually one horizontal scanning period) shouldbe taken into account. In addition, errors may occur depending on thescanning conditions. Thus, the above description is merely provided forconvenience in making the explanation easy and there is no limitationthereto.

For example, it is possible that pixel 16 is programmed with a 10-foldcurrent (N=10) and EL device is caused to light for a 1/5 period. Inthis case EL device 15 lights at a two-fold luminance (10/5=2).Alternatively, it is possible that pixel 16 is programmed with atwo-fold current (N=2) and EL device 15 is caused to light for a 1/4period. In this case EL device 15 lights at a 0.5-fold luminance(2/4=0.5). That is, according to the present invention, a pixel isprogrammed with an N-fold current (N is not equal to 1) and a displaywhich is not in a constant lighting state (i.e. 1/1, which does notmeans intermittent driving) is realized. In a wider sense, the presentinvention provides a driving method which includes cutting off feedingof current to EL device 15 at least once in a one-frame (or one-field)period. The present invention also provides a driving method whichincludes programming pixel 16 with a current higher than thepredetermined value while performing intermittent display necessarily.

Organic (or inorganic) EL display apparatus involve a problem essentialto their display method which is basically different from the displaymethod applied to such display apparatus as a CRT adapted to display animage as an aggregate of line displays provided by means of an electrongun. Since such an EL display apparatus is configured to hold a current(or a voltage) written to a pixel for a one-F (one-field or one-frame)period. This configuration gives rise to a problem of a blurred outlineof an image if it is displayed in a motion picture display state.

According to the present invention, EL device 15 is fed with a currentfor only a 1F/N period of a one-frame period and is not fed with acurrent for the rest of the frame period (1F(N−1)/N). Consideration isgiven to the case where one spot of the screen driven according to thisdriving method is observed. In this display state, a display based onimage data and a black display (non-lighting state) alternate with eachother on a 1F basis. That is, such a display based on image data appearsat time intervals (intermittent display). When a display based on motionpicture data is realized by such intermittent display driving, the imagehas no blurred outline, which means that a display of high quality isrealized. Thus, the intermittent display method can realize a motionpicture display close to that realized by a CRT. Further, since the mainclock used in the circuit is a conventional one in spite of intermittentdisplay, no increase occurs in the power consumption of the circuit.

In the case of a liquid crystal display panel, image data (voltage)based on which light modulation is performed is held in the liquidcrystal layer. Therefore, data applied to the liquid crystal layer needsto be rewritten in order to insert a black display. For this reason, itis required that the value of the clock for operating source driver 14be made higher while source signal line 18 applied with image data andblack display data alternately. Accordingly, the value of the main clockof the circuit needs to be raised in order to realize insertion of black(intermittent display of a black display or the like.) In addition,image memory for extending the time axis is also needed.

In a pixel configuration of the EL display panel of the presentinvention as shown in FIG. 1, 2 or 38 or the like, image data is held inthe capacitor 19. A current corresponding to the terminal voltage ofthis capacitor 19 is passed through EL device 15. Thus, image data isnot held in a light modulation layer as in the liquid crystal displaypanel.

According to the present invention, the current to be passed through ELdevice 15 is controlled by merely turning on/off switching transistor 11d or 11 e or the like. That is, even when the current Iw passing throughEL device 15 is cut off, image data is held as it is in the capacitor19. Therefore, when the switching device 11 d or the like is turned offat the next timing to feed EL device 15 with a current, this current hasa current value equal to that of the current passed just before. Thepresent invention does not need to raise the main clock of the circuiteven when insertion of black (intermittent display of a black display orthe like) is to be made. Nor does the present invention need to extendthe time axis and, hence, image memory therefor is not needed either.Organic EL device 15 requires a shortened time for the device 15 to emitlight from the time when it is fed with current and hence is responsiveat a high speed. For this reason, the present invention is suitable formotion picture display and is capable of solving the motion picturedisplay problem which is essential to display panels of the conventionaldata holding type (liquid crystal display panel, EL display panel, andthe like) by intermittent display.

In the case of a large-sized display apparatus having an increasedsource capacitance, the source current should be increased 10 times ormore. Generally, when the source current value is increased N times, itis sufficient to set the conducting period for gate signal line 17 b(transistor 11 d) to 1F/N. By so doing, the present invention isapplicable to television sets, monitoring display apparatus, and thelike.

The driving method according to the present invention will be describedmore specifically with reference to the drawings. The parasitic capacityof source signal line 18 is produced due to the coupling capacitancebetween adjacent source signal lines 18, the capacitance of the bufferoutput of source driver IC (circuit), the capacitance at a crossingpoint between gate signal line 17 and source signal line 18, and thelike. Such a parasitic capacity is usually 10 pF or more. In the case ofvoltage-based driving, driver IC 14 applies a voltage to source signalline 18 with a low impedance and, hence, some increase in the parasiticcapacity does not raise any driving problem.

However, in the case of current-based driving, image display of a blacklevel, in particular, requires programming of capacitor 19 of a pixelwith a faint current of 20 nA or lower. For this reason, when theparasitic capacity takes place as having a value more than apredetermined value, the parasitic capacity cannot be charged/dischargedwithin the time required for one pixel row to be programmed. (The timerequired is usually a 1H period or shorter but is not limited theretosince two pixel rows may be programmed at a time.) If charge/dischargeis impossible within a 1H period, writing to a pixel is insufficientand, hence, display with a desired resolution cannot be realized.

In the case of the pixel configuration shown in FIG. 1, a programmingcurrent Iw passes through source signal line 18 during current-basedprogramming as shown in FIG. 3( a). The current Iw is passed throughtransistor 11 a to set (program) a voltage of capacitor 19 so that thevoltage for causing the current Iw to pass is held. At this timetransistor 11 d is in an open state (off-state).

In turn, transistors 11 c and 11 b are turned off and transistor 11 doperates in the period for feeding EL device 15 with a current as shownin FIG. 3( b). Specifically, off-voltage (Vgh) is applied to gate signalline 17 a to turn transistors 11 b and 11 c off. On the other hand,on-voltage (Vgl) is applied to gate signal line 17 b to turn transistor11 d off.

Now, assuming that the current Iw is 10 times as high as a current (of apredetermined value) to be passed conventionally, a current passingthrough EL device 15 in FIG. 3( b) is also 10 times as high as thepredetermined value. Accordingly, EL device 15 emits light at aluminance 10 times as high as a predetermined value. That is, thedisplay luminance B of the display panel becomes higher with increasingmagnification N, as shown in FIG. 12. Therefore, the luminance and themagnification are proportional to each other. With 1/N driving, on theother hand, the luminance and the magnification are inverse proportionto each other.

If transistor 11 d is caused to assume on-state for only 1/N of the timeperiod for which transistor 11 assumes on-state conventionally and toassume off-state for the rest ((N−1)/N) of the time period, the meanluminance throughout 1F becomes a predetermined luminance. This displaystate is close to a display state of a screen scanned with an electrongun in a CRT. The difference therebetween resides in that the regiondisplaying an image or the lighting region is 1/N of the whole screen(which is equal to 1.) (The lighting region in the CRT corresponds toone pixel row (one pixel in a strict sense).)

In the present invention, 1F/N image display region 53 shifts from theupper side to the lower side of screen 50, as shown in FIG. 13( b). ELdevice 15 is fed with current for only a 1F/N period and is not fed withcurrent for the rest (1F·(N−1)/N) of the period. Therefore, each pixeldisplays intermittently. However, the image is seen to be retained athuman eyes through afterimage and, hence, the whole screen is seen todisplay uniformly.

It should be noted that written pixel row 51 a forms a non-lightingdisplay 52 a, as shown in FIG. 13. However, this occurs in the pixelconfigurations shown in FIGS. 1 and 2. Such a written pixel row 51 a mayassume a lighting state in the current mirror pixel configuration shownin FIG. 38 or the like. In the present description, however, the pixelconfiguration shown in FIG. 1 is mainly exemplified for easyexplanation. The method illustrated in FIG. 13 or 16 or the like, whichincludes programming with a current higher than the predetermineddriving current Iw and intermittent driving, will be referred to as anN-fold pulse driving method.

In this display state, a display based on image data and a black display(non-lighting state) alternate with each on a 1F basis. That is, such adisplay based on image data appears at time intervals (intermittentdisplay). Since liquid crystal display panels (and EL display panelsother than the EL display panels of the present invention) areconfigured to hold data at pixels for a 1F period, an image on a motionpicture display cannot keep up with image data changing, resulting inblurred motion picture (blurred image outline). According to the presentinvention, however, an image is displayed intermittently and, hence,satisfactory display state with no blurred outline can be realized.Thus, the intermittent display method can realize a motion picturedisplay close to that realized by a CRT.

The timing chart of such intermittent display is shown in FIG. 14. Thepixel configuration shown in FIG. 1 is exemplified in the presentinvention unless otherwise particularly specified. As seen from FIG. 14,in each selected pixel row (selecting period is 1H), gate signal line 17b is under application of off-voltage (Vgh) (see FIG. 14( b)) while gatesignal line 17 a is being applied with on-voltage (Vgl) (see FIG. 14(a).) During this period, EL device 15 is not fed with current (in anon-lighting state). In an unselected pixel row, on the other hand, gatesignal line 17 a is under application of off-voltage (Vgh) and gatesignal line 17 b is under application of on-voltage (Vgl). During thisperiod, EL device 15 is fed with current (in a lighting state). In thelighting state, EL device lights at a luminance N times as high as apredetermined value (N·B) for a time period of 1F/N. Thus, a meansdisplay luminance of the display panel throughout a 1F period can befound from the equation: (N·B)×(1/N)=B (predetermined luminance).

FIG. 15 illustrates an embodiment in which the operation illustrated inFIG. 14 is applied to pixel rows. Specifically, voltage waveforms to beapplied to respective gate signal lines 17 are shown. Each voltagewaveform comprises off-voltage Vgh (H level) and on-votlage Vgl (Llevel). Additional numerals such as (1) and (2) indicate the row numbersof selected pixel rows.

In FIG. 15, when gate signal line 17 a(1) is selected (at voltage Vgl),a programming current is passed through source signal line 18 fromtransistor 11 a of the selected pixel row toward source driver 14. Thisprogramming current is N times as high as a predetermined value.(Description is made with N=10 for easy explanation. Since thepredetermined value is the value of a data current causing an image tobe displayed, the predetermined value is not a fixed value unless whiteraster display is given.) Accordingly, capacitor 19 is programmed sothat a 10-fold current will pass through transistor 11 a. When pixel row(1) is in the selected state, gate signal line 17 b(1) of the pixelconfiguration of FIG. 1 is under application of off-voltage (Vgl), thuspreventing current from passing through EL device 15.

After lapse of 1H, gate signal line 17 a(2) is selected (at voltage Vgl)and a programming current is passed through source signal line 18 fromtransistor 11 a of the selected pixel row toward source driver 14. Thisprogramming current is N times as high as a predetermined value.(Description is made with N=10 for easy explanation.) Accordingly,capacitor 19 is programmed so that a 10-fold current will pass throughtransistor 11 a. When pixel row (2) is in the selected state, gatesignal line 17 b(2) of the pixel configuration of FIG. 1 is underapplication of off-voltage (Vgl), thus preventing current from passingthrough EL device 15. On the other hand, the preceding pixel row (1)assumes a lighting state because gate signal line 17 a(1) and gatesignal line 17 b(1) of pixel row (1) are applied with off-voltage (Vgh)and on-voltage (Vgl), respectively.

After lapse of another 1H, gate signal line 17 a(3) is selected and gatesignal line 17 b(3) is applied with off-voltage (Vgh) to prevent currentfrom passing through EL device 15 of pixel row (3). On the other hand,the preceding pixel rows (1) and (2) assume the lighting state becausegate signal lines 17 a(1) and 17 a(2) thereof are applied withoff-voltage (Vgl) and gate signal lines 17 b(1) and 17 b(2) thereof areapplied with on-voltage (Vgl).

The above-described operation is synchronized with a 1H synchronizingsignal. With the driving method of FIG. 15, however, a 10-fold currentpasses through El device 15 and, accordingly, display screen 50 displaysan image at a luminance having about a 10-fold value. Of course, it isneedless to say that the programming current should be decreased to 1/10in order to realize a display at the predetermined luminance. With sucha 1/10 current, however, insufficient writing occurs due to parasiticcapacity and the like. The basic concept of the present invention isthat programming is made with a high current to avoid such insufficientwriting while black display 52 is inserted to obtain the predeterminedluminance.

An important feature of the driving method of the present inventionresides in that a current higher than the predetermined current iscaused to pass through EL device 15 thereby sufficientlycharging/discharging the parasitic capacity of source signal line 18.Therefore, EL device 15 need not necessarily be fed with a current Ntimes as high as the predetermined current. For example, a configurationmay be employed such that a current path is formed in parallel with ELdevice 15 (specifically, a dummy EL device is formed which has beensubjected to such processing as to prevent the dummy EL device fromemitting light, for example, formation of a light-shielding filmthereover) and a current is dividedly fed to the dummy EL device and ELdevice 15. When the signal current is 0.2 μA for example, theprogramming current adjusted to 2.2 μA is passed through transistor 11a. Of this current, the signal current of 0.2 μA is fed to EL device 15while the remaining current of 2.0 μA fed to the dummy EL device. Such adriving method is exemplified. That is, dummy pixel row 281 shown inFIG. 27 is made constantly selected. The dummy pixel row is made to failto emit light or formed with a light-shielding film to prevent emissionof light from being recognized visually.

With such an arrangement, programming can be made so that a current Ntimes as high as the predetermined current will pass through drivingtransistor 11 a by increasing the current to pass through source signalline 18 N times, while at the same time a current sufficiently lowerthan the N-fold current can be passed through EL device 15. Theabove-described method does not need to provide non-lighting region 52shown in FIG. 5 and hence can allow the whole display region 50 to beused as image display region 53.

FIG. 13( a) illustrates a written state of display screen 50. Referencecharacter 51 a used in FIG. 13( a) designates a written pixel row.Source driver 14 feeds the programming current to each source signalline 18. In FIG. 3 or the like, writing is made to a single pixel row ina 1H period. However, there is no particular limitation to 1H but it ispossible to employ a 0.5H period or a 2H period. Though the programmingcurrent is written to source signal line 18 according to the abovedescription, the present invention is not limited to such acurrent-based programming method but may employ a voltage-basedprogramming method (illustrated in FIG. 62 or the like) in which sourcesignal line 18 is written with a voltage.

In FIG. 13( a), when gate signal line 17 a is selected, transistor 11 ais programmed with a current passing through source signal line 18. Atthat time, gate signal line 17 b is applied with off-voltage and, as aresult, EL device 15 is not fed with a current. This is because whentransistor 11 d is in on-state, a capacitance component of EL device 15is seen from source signal line 18 and capacitor 19 cannot sufficientlyaccurately be programmed with current because of the influence of thecapacitance. Accordingly, in the configuration of FIG. 1 for example, apixel row written with current forms non-lighting region 52, as shown in13(b).

If programming is made with an N-fold current (here, N=10 as describedearlier), the luminance of the screen is increased 10 times. Therefore,non-lighting region 52 should cover 90% of display region 50.Specifically, if an image display region has 220 horizontal scanninglines (S=220) in Quarter Common Intermediate Format (QCIF), 22 linesshould form display region 53, with the rest (220−22=198) formingnon-display region 52. Generally speaking, if the number of horizontalscanning lines (the number of pixel rows) is S, an S/N region is used asdisplay region 53 which is caused to emit light at an N-fold luminance.This display region 53 is scanned vertically of the screen. Thus, theremaining S(N−1)/N region is used as non-lighting region 52. Thisnon-lighting region forms a black display (luminescenceless region.)Such a luminescenceless region 52 is realized by turning transistor 11 doff. Though the display region 53 has been described to light at anN-fold luminance, it is needless to say that the value of N can becontrolled by brightness adjustment or gamma adjustment, as a matter ofcourse.

In the above-described embodiment, non-lighting region 52 should cover90% of display region 50 because if programming is made with an N-foldcurrent, the luminance of the screen is increased 10 times. However,this feature is not limited to an arrangement where R, G and B pixelsform non-lighting regions 52 in the same manner. For example, theproportion of non-display region 52 may be varied depending on R, G andB; for example, R pixel provides non-lighting region 52 covering ⅛ ofdisplay region 50, G pixel provides non-lighting region 52 covering ⅙ ofdisplay region 50, and B pixel provides non-lighting region 52 covering1/10 of display region 50. Alternatively, it is possible to employ anarrangement such as to adjust non-lighting region 52 (or lighting region53) in individual R, G and B pixels. To realize these arrangements, gatesignal lines 17 b for respective of R, G and B need to be provided. Bymaking individual adjustment of R, G and B possible, it becomes possibleto control white balance as well as to ease color balance adjustment ateach gray level (see FIG. 41.)

As shown in FIG. 13( b), pixel rows including written pixel row 51 aform non-lighting region 52, while an S/N region (which is 1F/N in termsof time) in a screen portion above written pixel row 51 a forms lightingregion 53. (In the case of scanning upwardly from the lower side of thescreen, lighting region 53 is situated on the opposite side.) In thisimage display state, band-like display region 53 shifts downwardly fromthe upper side of the screen.

In the display shown in FIG. 13, one display region 53 shifts downwardlyfrom the upper side of the screen. If the frame rate is low, shifting ofdisplay region 53 is visually recognized. This is likely particularlywhen the viewer blinks his or her eyes or moves his or her face up anddown.

To solve this problem, display region 53 should be split into pluralsections as shown in FIG. 16. If the total sum of the areas of thesections is equal to the area of an S(N−1)/N region, the brightness ofthis display is equal to that of the display shown in FIG. 13. Displayregion 53 need not necessarily be split equally. Similarly, sections ofnon-display region 52 split need not necessarily be uniform.

By thus splitting display region 53 into plural sections, the screenprovides a display with reduced flitter. Thus, favorable image displayfree of flicker can be realized. Display region 53 may be split intosmaller sections. However, with finer splitting, the motion picturedisplay performance lowers.

FIG. 17 shows a voltage waveform applied to each gate signal line 17 andthe luminance of the EL device emitting light. As can be clearly seenfrom FIG. 17, the (1F/N) period for which gate signal line 17 b isapplied with Vgl is divided into plural subperiods (the number ofsubperiods is K.) That is, gate signal line 17 b is applied with Vgl fora 1F/(K/N) period K times. Such a control can inhibit the occurrence offlicker and realize image display with a low frame rate. It is alsopreferable to employ such an arrangement as to allow the number of suchimage divisions to be varied. For example, an arrangement is possiblesuch as to detect a change resulting from depressing of a brightnessadjuster switch or turning of a brightness adjuster volume and then varythe value of K. Another possible arrangement allows the user to adjustthe luminance. Yet another possible arrangement allows the user to varythe number of K depending on the details of or data on an image to bedisplayed manually or is capable of varying the number of Kautomatically.

While description has been made of the feature that the (1F/N) periodfor which gate signal line 17 b is applied with Vgl is divided intoplural subperiods (the number of subperiods is K) and gate signal line17 b is applied with Vgl for a 1F/(K/N) period K times, there is nolimitation to this feature. Gate signal line 17 b may be applied withVgl for the 1F/(K/N) period L times (L≠K). Thus, the present inventionhas the feature that an image is displayed by controlling the period(time) for which EL device 15 is fed with current. Therefore, the art ofrepeating the 1F/(K/N) period L times (L≠K) is included in the technicalconcept of the present invention. The luminance of image 50 can bevaried digitally by varying the value of L. For example, the differencebetween L=2 and L=3 corresponds to a 50% change in luminance (contrast).In splitting display region 53, the period for which gate signal line 17b is applied with Vgl is not necessarily constant.

The above-described embodiment is an embodiment in which display screen50 is turned on/off (into lighting state/non-lighting state) by cuttingof the current to be passed through EL device or passing the currentthrough EL device. That is, the embodiment is configured to passgenerally equal current through transistor 11 a plural times by thecharge held in capacitor 19. However, the present invention is notlimited thereto. The present invention may employ such a configurationas to turn display screen 50 on/off (into lighting state/non-lightingstate) by charging/discharging capacitor 19.

FIG. 18 shows a voltage waveform applied to each gate signal line 17 forrealizing the image display state shown in FIG. 16. The differencebetween FIG. 18 and FIG. 15 resides in the operation of gate signal line17 b. Gate signal line 17 b is turned on/off (with Vgl or Vgh) pluraltimes, the number of times corresponding to the number of split sectionsof the screen. Since other features are the same as the correspondingfeatures of FIG. 15, description thereof will be omitted.

Since the EL display apparatus assumes a completely non-lighting stateto provide a black display, a drop in contrast, which is essential tointermittent display performed by a liquid crystal display panel, doesnot occur. With the configuration shown in FIG. 1, intermittent displaycan be realized by merely on-off controlling transistor 11 d. With eachof the configurations shown in FIGS. 38 and 51, intermittent display canbe realized by merely on-off controlling transistor 11 e. This isbecause capacitor 19 stores image data. (The number of gray levels isinfinite since such stored image data is an analog value.) Specifically,each pixel 16 stores image data for a 1F period. Whether or not ELdevice 15 is to be fed with a current corresponding to image data storedin each pixel 16 is controlled by control over transistors 11 d and 11e. Thus, the above-described driving method is applicable not only tothe current-driven configuration but also to the voltage-drivenconfiguration. Stated otherwise, the driving method can realizeintermittent driving of a configuration where each pixel is adapted tostore a current to be passed through EL device 15 by turning on/off thedriving transistor 11 on the current path between EL devices 15.

It is critical to maintain the terminal voltage of capacitor 19. This isbecause when the terminal voltage of capacitor 19 varies (i.e.,capacitor 19 is charged/discharged) during a one-field (frame) period,the luminance of the screen varies, which results in flitter (flicker orthe like) when the frame rate is lowered. It is required that thecurrent to be passed through EL device 15 during a one-frame (field)period should not lower to 65% or less. The value of 65% means thatassuming the first current written to pixel 16 and passed through ELdevice 15 is 100%, the current to be passed through EL device 15 justbefore writing to the pixel 16 in the next frame (or field) is set to65% or more.

In the configuration shown in FIG. 1, the number of transistors 11forming one pixel is not varied irrespective of whether or notintermittent display is realized. That is, satisfactory current-basedprogramming is realized by eliminating the influence of the parasiticcapacity of source signal line 18 without changing the pixelconfiguration. In addition, picture motion display close to thatprovided by a CRT can be realized.

Since the clock for operating gate driver 12 is sufficiently slow ascompared to the clock for operating source driver 14, the main clock ofthe circuit does not rise. Further, the value of N can be varied easily.

It is possible that the image displaying direction (image writingdirection) at the first field (frame) is the direction from the upperside to the lower side of the screen while the image displayingdirection at the second field (frame) is the direction from the lowerside to the upper side of the screen. That is, the downwardly displayingdirection and the upwardly displaying direction may alternate with eachother repeatedly.

It is also possible that the image displaying direction at the firstfield (frame) is the direction from the upper side to the lower side ofthe screen and after the whole screen has been temporarily turned into ablack display (into a non-display state), the image displaying directionis switched to the direction from the lower side to the upper side ofthe screen at the subsequent second field (frame). The whole screen maypresent a black display once.

Though the aforementioned driving method has been described to performthe writing to the screen in the direction from the upper side to thelower side of the screen or from the lower side to the upper side of thescreen, there is no limitation to this feature. It is possible that thedirection of writing to the screen from the upper side to the lower sideor from the lower side to the upper side is fixed whereas non-displayregion 52 shifts in the direction from the upper side to the lower sideof the screen at a first field (frame) while shifting in the directionfrom the lower side to the upper side of the screen at a subsequentsecond field. It is also possible that one frame is divided into threefields, the first, second and third ones of which are allocated to R, Band G, respectively, and, hence, three fields constitute one frame. Itis also possible that R, G and B are switched one to another on a onehorizontal scanning period (1H) basis. The above-described matters holdtrue for other embodiments of the present invention.

Non-display region 52 need not necessarily assume a completelynon-lighting state. There arises no practical problem even when faintluminescence or faint image display occurs. Such faint luminescence orfaint image display should be construed as a region having a lowerdisplay luminance than image display region 53. The “non-display region52” is meant to include even the case where one or two of R, G and Bimage display pixels are in the non-display state.

Basically speaking, with the luminance (brightness) of display region 53being maintained to a predetermined value, the luminance of screen 50rises with increasing area of display region 53. For example, withdisplay region 53 having a luminance of 100 (nt), an increase in theproportion of display region 53 relative to the whole screen 50 from 10%to 20% raises the screen luminance twice. Thus, the display luminance ofthe screen can vary with varying area of display region 53 in the wholescreen 50.

The area of display region 53 can be set as desired by controlling datapulse (ST2) to be fed to shift register 61. Further, the display stateshown in FIG. 16 and the display state shown in FIG. 13 can be switchedto each other by varying the data pulse input timing and the data pulseinput cycle. An increase in the number of data pulses per 1F periodcauses screen 50 to become brighter, whereas a decrease in the number ofdata pulses causes screen 50 to become darker. Continuous application ofdata pulses results in the display state shown in FIG. 13, whileintermittent inputting of data pulses results in the display state shownin FIG. 16.

FIG. 19( a) illustrates a method of brightness adjustment applicable tothe case where display region 53 is continuous as shown in FIG. 13. Thescreen 50 at FIG. 19( a 1) has the highest display luminance. Thedisplay luminance of the screen 50 at FIG. (a2) is next to the highest,whereas that of the screen 50 at FIG. (a3) is the lowest. The change instate from FIG. 19( a 1) to FIG. 19( a 3) and vice versa can be easilyrealized by control over the shifter register 61 of gate driver 12 andthe like as described above. At that time, the voltage Vdd in FIG. 1need not be varied. That is, the luminance of display screen 50 can bevaried without varying the supply voltage. The gamma characteristic ofthe screen does not vary at all with the change in state from FIG. 19( a1) to FIG. 19( a 3). Thus, the contrast and the gray scalecharacteristic of a displayed image are maintained irrespective of theluminance of screen 50. This is an effect characteristic of the presentinvention. With the conventional screen luminance adjustment, the grayscale performance is low when the luminance of screen 50 is low.Specifically, though a 64-level gray scale display can be realized at ahigh luminance display, the number of displayable gray levels isdecreased to a half or less at a low luminance display in most cases. Incontrast, the driving method of the present invention is capable ofrealizing the maximum 64-level gray scale display without dependence onthe display luminance of the screen.

FIG. 19( b) illustrates a method of brightness adjustment applicable tothe case where display region 53 is dispersed as shown in FIG. 16. Thescreen 50 at FIG. 19( b 1) has the highest display luminance. Thedisplay luminance of the screen 50 at FIG. (b2) is next to the highest,whereas that of the screen 50 at FIG. (b3) is the lowest. The change instate from FIG. 19( b 1) to FIG. 19( b 3) and vice versa can be easilyrealized by control over the shifter register 61 of gate driver 12 andthe like as described above. If display region 53 is dispersed as shownin FIG. 19( b), flicker does not occur even at a low frame rate.

In order to further lessen the occurrence of flicker at a low framerate, display region 53 should be dispersed more finely as shown in FIG.19( c). In this case, however, the motion picture display performancelowers. Therefore, the driving method illustrated in FIG. 19( a) issuitable for motion picture display. The driving method illustrated inFIG. 19( c) is suitable for the case where a stationary image isdisplayed with low power consumption demanded. Switching from the FIG.19( a) method to the FIG. 19( c) method can be easily realized bycontrol over shift register 61.

FIG. 20 is an explanatory view illustrating another embodiment forincreasing the current to be fed to source signal line 18. Thisembodiment is a method of significantly improving insufficient writingwith current, which basically comprises selecting plural pixel rows at atime and charging/discharging the parasitic capacity of source signalline 18 and the like with a current which is the sum of currentsrequired by the plural pixel rows. Since plural pixel rows are selectedat a time, the current for driving one pixel can be decreased. Hence,the current to be fed to EL device 15 can be decreased. Here, for easyexplanation, the case of N=10 (in which a 10-fold current is passedthrough source signal line 18) will be described as an example.

As shown in FIG. 20, K pixel rows are selected according to the presentinvention. Source signal line 18 is applied with a current N times ashigh as a predetermined current from source driver 14. Each pixel isprogrammed with a current N/K times as high as the current to be passedthrough the EL device 15. The time period for which the EL device 15 isfed with the current is set to K/N of a one-frame (field) period. Such adriving method makes it possible to charge/discharge the parasiticcapacity of source signal line 18 sufficiently as well as to obtainsatisfactory resolution and a predetermined luminance of emission.

Specifically, EL device 15 is fed with current for K/N of a one-frame(field) period and is not fed with current for the rest (1F(N−1)K/N) ofthe one-frame period. In this display state, a display based on imagedata and a black display (non-lighting state) alternate with each otherrepeatedly 1F by 1F. That is, such a display based on image data appearsat time intervals (intermittent display). Thus, a motion picture displayof high quality with no blurred outline can be realized. Further, sincesource signal line 18 is driven with an N-fold current, the parasiticcapacity does not affect the display and, hence, the driving method ofthe present invention is applicable to high-resolution display panels.

FIG. 21 is an explanatory diagram of driving voltage waveforms used forrealizing the driving method illustrated in FIG. 20. In this figure, asignal waveform comprises off-voltage Vgh (H level) and on-voltage Vgl(L level). The numeral added to each signal line, such as (1), (2) or(3), indicates the row number of each pixel row. It should be noted thata QCIF display panel has 220 rows while a VGA panel has 480 rows.

In FIG. 21, when gate signal line 17 a(1) is selected (at voltage Vgl),a programming current is passed through source signal line 18 fromtransistor 11 a of the selected pixel row toward source driver 14. Foreasy explanation, description will be made of the case where pixel row51 a to be written is the first pixel row.

The programming current to be passed through source signal line 18 is Ntimes as high as a predetermined value. (Description is made with N=10for easy explanation. Since the predetermined value is the value of adata current causing an image to be displayed, the predetermined valueis not a fixed value unless a white raster display is provided.)Further, description will be made of the case where five pixel rows areto be selected at a time (K=5.) Accordingly, the capacitor 19 of onepixel is programmed so that, ideally, a 2-fold current (N/K=10/5=2) willpass through transistor 11 a.

When the written pixel row is the first pixel row (1), gate signal lines17 a(1) to 17 a(5) are in the selected state. That is, the switchingtransistors 11 b and 11 c of each of pixels rows (1) to (5) are inon-state. Also, gate signal line 17 b is in reversed phase with gatesignal line 17 a. Accordingly, the switching transistor 11 d of each ofthe pixel rows (1) to (5) is in off-state, thus preventing current frompassing through EL devices 15 of the associated pixel row. That is,these EL devices are in the non-lighting state 52.

Ideally, the transistors 11 a of five pixels each pass a current of Iw×2through source signal line 18. (That is, a current ofIw×2×N=Iw×2×5=Iw×10 is passed through source signal line 18. Therefore,assuming that the current to be passed through source signal line 18 inthe case where the N-fold pulse driving method of the present inventionis not employed is the predetermined current Iw, a current 10 times ashigh as Iw is to be passed through source signal line 18.)

The operation (driving method) described above causes the capacitor 19of each pixel 16 to be programmed with a 2-fold current. Here,description is made on the assumption that transistors 11 a are uniformin characteristics (Vt and S value) for easy understanding.

Since the number of pixel rows selected at a time is five (K=5), fivedriving transistors 11 a operate. That is, a 2-fold (10/5=2) currentpasses through transistor 11 a per pixel. Source signal line 18 is fedwith a current as the sum of programming currents for the fivetransistors 11 a. For example, assuming that the current to beconventionally passed through pixel row 51 a to be written is Iw, acurrent of Iw×10 is to be passed through source signal line 18 accordingto the present invention. Pixel rows 51 b to be written with image dataafter writing to pixel row (1) are now used as auxiliary pixel rows forincreasing the amount of current to be fed to source signal line 18.However, there arises no problem because the pixel rows 51 b will bewritten with correct image data thereafter.

Therefore, the four pixel rows 51 b provide the same display as thepixel row 51 a during a 1H period. For this reason, at least the writtenpixel row 51 a and the pixel rows 51 b selected for increasing thecurrent are made to assume the non-lighting state 52. However, suchpixel rows in a current mirror pixel configuration as shown in FIG. 38or other pixel configurations adapted for voltage-based programming maybe made to assume the lighting state.

After lapse of 1H, gate signal line 17 a(1) assumes the unselected statewhile gate signal line 17 b is applied with on-voltage (Vgl). At thesame time, gate signal line 17 a(6) is selected (applied with Vglvoltage) and transistor 11 a of the selected pixel row (6) passes theprogramming current through source signal line 18 toward source driver14. Such an operation allows pixel row (1) to hold regular image data.

After lapse of another 1H, gate signal line 17 a(2) assumes theunselected state while gate signal line 17 b is applied with on-voltage(Vgl). At the same time, gate signal line 17 a(7) is selected (appliedwith Vgl voltage) and transistor 11 a of the selected pixel row (7)passes the programming current through source signal line 18 towardsource driver 14. Such an operation allows pixel row (2) to hold regularimage data. By performing the above-described operation with scanningshifting pixel row by pixel row, one screen is wholly rewritten.

With the driving method of FIG. 20, each pixel is programmed with a2-fold current (voltage) and, hence, ideally the EL device 15 of eachpixel emits light at a 2-fold luminance. Therefore, the luminance of thedisplay screen is twice as high as the predetermined value. In order forthe display screen to display at the predetermined luminance, a regionincluding written pixel row 51 and occupying ½ of display region 50should be used as non-display region 52.

As in the case of FIG. 13, when one display region 53 shifts downwardlyfrom the upper side of the screen as shown in FIG. 20, the shifting ofdisplay region 53 is visually recognized if the frame rate is low. Thisis likely particularly when the viewer blinks his or her eyes or moveshis or her face up and down.

To solve this problem, display region 53 should be split into pluralsections as shown in FIG. 22. If the total sum of the areas of thesesections is equal to the area of an S(N−1)/N region, the brightness ofthis display is equal to that of the display provided without splittingof display region 53.

FIG. 23 shows a voltage waveform applied to each gate signal line 17.The difference between FIG. 21 and FIG. 23 resides in the operation ofgate signal line 17 b. Gate signal line 17 b is turned on/off (with Vgland Vgh) plural times, the number of times corresponding to the numberof split sections of the screen. Since other features are substantiallythe same as or analogous to the corresponding features of FIG. 21,description thereof will be omitted.

By thus splitting display region 53 into plural sections, the screenprovides a display with reduced flitter. Thus, satisfactory imagedisplay free of flicker can be realized. Display region 53 may be splitinto smaller sections. With finer splitting, flicker can be morereduced. Since the responsiveness of EL device 15 is particularly high,the display luminance will not lower even if EL device 15 is turnedon/off at a time interval shorter than 5 μsec.

In the driving method of the present invention, EL device 15 can beon-off controlled by turning on/off the signal to be applied to gatesignal line 17 b. For this reason, such control can be achieved with aclock having a low frequency on the KHz order. Further, image memory orthe like is not needed for inserting a black display (i.e., non-displayregion 52). Therefore, the driving circuit or method of the presentinvention can be implemented with reduced cost.

FIG. 24 illustrates the case where the number of pixel rows to beselected at a time is two. According to the results of study made by theinventors et al., the method including selection of two pixel rows at atime realized practical display uniformity when applied to displaypanels formed by the low temperature polysilicon technology. Presumably,this is because driving transistors 11 a of adjacent pixels were veryuniform in their characteristics. Good results were obtained byperforming striped laser irradiation parallel with source signal line 18in laser annealing.

This is because portions of a semiconductor film in a region annealed atthe same time are uniform in characteristics. Stated otherwise, this isbecause a semiconductor film is formed uniformly in a striped regionirradiated with laser light and transistors formed using thissemiconductor film are substantially uniform in Vt and mobility. Thus,pixels arranged along source signal line 18 (i.e., a pixel columnextending vertically of the screen) are made substantially uniform incharacteristics by irradiation with striped laser shot in parallel withthe source signal line 18 forming direction and shifting the irradiatingposition. Therefore, when plural pixel rows are turned on at a time soas to be programmed with current, the plural pixel rows selected at atime are programmed with a substantially equal current having a valuewhich is the quotient obtained by dividing the programming current bythe number of the selected pixel rows. Thus, it is possible to realizecurrent-based programming with a current value close to a target value,hence, realize a uniform display. For this reason, the laser shotdirection and the driving method illustrated in FIG. 24 or the likeprovide a synergetic effect.

As described above, transistors 11 a of vertically arranged pixels aremade substantially uniform in characteristics by making the direction oflaser shot substantially coincident with the direction in which sourcesignal line 18 is formed, thus resulting in satisfactory current-basedprogramming. (In this case, transistors 11 a of horizontally arrangedpixels need not necessarily be uniform in characteristics.) Theoperation thus described is performed, while the position of pixel rowsto be selected is shifted one pixel row by one pixel row or plural pixelrows by plural pixel rows in synchronism with 1H (one-horizontalperiod). Though the laser shot direction described is made parallel withsource signal line 18 according to the above description, the presentinvention is not limited to the laser shot direction parallel withsource signal line 18. This is because irradiation with laser shot in anoblique direction with respect to source signal line 18 allowstransistors 11 a of vertically arranged pixels along one source signalline 18 to be made substantially uniform in characteristics. Therefore,the “irradiation with laser shot parallel with source signal line” ismeant to form any adjacent pixels to be arranged along the wiringdirection of source signal line 18 (in the vertical direction) in amanner to locate them within one laser irradiation region. The “sourcesignal line 18” generally means wiring for transmission of programmingcurrents or voltages serving as image signals.

According to the above-described embodiment of the present invention,the position of pixel rows to be written is shifted 1H by 1H. However,the present invention is not limited to this feature. It is possible toshift the position 2H by 2H or on the basis of more pixel rows.Alternatively, shifting may be performed based on any unit time. Theshifting time interval may be varied with varying position on thescreen. For example, it is possible that the shifting time interval isshortened at a central portion of the screen and prolonged at upper andlower portions of the screen. Also, the shifting time interval may bevaried frame by frame. The present invention is not limited to selectionof plural pixel rows arranged adjacent to each other. For example, it ispossible to select pixel rows located across one intervening pixel row.Specifically, a driving method may be employed such that the first andthird pixel rows are selected in the first horizontal scanning period,the second and fourth pixel rows selected in the second horizontalscanning period, the third and fifth pixel rows selected in the thirdhorizontal scanning period, and the fourth and sixth pixel rows selectedin the fourth horizontal scanning period. Of course, the technical scopeof the present invention includes a driving method such as to select thefirst, third and fifth pixel rows in the first horizontal scanningperiod. It is, of course, possible to select pixel row positions acrossplural intervening pixel rows.

It is needless to say that the combination of the feature of the lasershot direction setting and the feature of the simultaneous selection ofplural pixel rows is applicable not only to the pixel configurationsshown in FIGS. 1, 2 and 32 but also to other current-driven pixelconfigurations as shown in FIGS. 38, 42 and 50 including the currentmirror pixel configuration shown in FIG. 38. The combination is alsoapplicable to voltage-driven pixel configurations as shown in FIGS. 43,51, 54 and 62. This is because if the transistors of pixels arrangedadjacent to each other vertically are uniform in characteristics,satisfactory voltage-based programming can be realized with a voltageapplied to a common source signal line 18.

When the first pixel row is written in the configuration shown FIG. 24,gate signal lines 17 a(1) and 17 a(2) are selected (see FIG. 25.) Thatis, the switching transistors 11 b and transistors 11 c of pixel rows(1) and (2) are in on-state. Each gate signal line 17 b is in reversedphase with each gate signal line 17 a. Accordingly, the switchingtransistors 11 d of at least the pixel rows (1) and (2) are inoff-state, thus preventing current from passing through EL devices 15 ofthe associated pixel rows. That is, these pixel rows are in thenon-lighting state 52. It should be noted that in the arrangement shownin FIG. 24, display region 53 is split into five sections in order toreduce the occurrence of flicker.

Ideally, the transistors 11 a of two pixels (pixel rows) each pass acurrent of Iw×5 (N=10) through source signal line 18. (That is, sinceK=2, a current of Iw×K×5=Iw×10 is passed through source signal line 18.)Therefore, the capacitor 19 of each pixel 16 is programmed with a 5-foldcurrent.

Since the number of pixel rows selected at a time is two (K=2), twodriving transistors 11 a operate. That is, a 5-fold (10/2=5) currentpasses through each transistor 11 a. Source signal line 18 is fed with acurrent as the sum of programming currents for the two transistors 11 a.

For example, pixel row 51 a to be written is fed with current Id, whichis to be conventionally fed to pixel row 51 a, while source signal line18 is fed with a current of Iw×10. However, there arises no problembecause the pixel row 51 b will be written with regular image datathereafter. The pixel row 51 b provides the same display as the pixelrow 51 a during a 1H period. For this reason, at least the written pixelrow 51 a and the pixel row 51 b selected for increasing the current aremade to assume the non-lighting state 52.

After lapse of 1H, gate signal line 17 a(1) assumes the unselected statewhile gate signal line 17 b is applied with on-voltage (Vgl). At thesame time, gate signal line 17 a(3) is selected (applied with Vglvoltage) and the transistor 11 a of the selected pixel row (3) passesthe programming current through source signal line 18 toward sourcedriver 14. Such an operation allows pixel row (1) to hold regular imagedata.

After lapse of another 1H, gate signal line 17 a(2) assumes theunselected state while gate signal line 17 b is applied with on-voltage(Vgl). At the same time, gate signal line 17 a(4) is selected (appliedwith Vgl voltage) and the transistor 11 a of the selected pixel row (4)passes the programming current through source signal line 18 towardsource driver 14. Such an operation allows pixel row (2) to hold regularimage data. By performing the above-described operation with scanningshifting pixel row by pixel row, one screen is wholly rewritten. (Ofcourse, scanning may be shifted plural pixel rows by plural pixel rows.For example, a pseudo-interlaced driving method will shift scanning tworows by two rows. In terms of image display, there will be some caseswhere the same image is written to plural pixel rows.)

Similarly to the case of FIG. 16, the driving method illustrated in FIG.24 programs each pixel with a 5-fold current (voltage) and, hence,ideally the EL device 15 of each pixel emits light at a 5-foldluminance. Therefore, the luminance of display region 53 is 5 times ashigh as the predetermined value. In order for the display region 53 todisplay at the predetermined luminance, a region including written pixelrows 51 and occupying ⅕ of display screen 50 should be used asnon-display region 52.

As shown in FIG. 27, two pixel rows to be written 51 (51 a and 51 b) areselected and such selection is made sequentially from the upper side tothe lower side of screen 50. (See FIG. 26 also. In FIG. 26, pixel rows16 a and 16 b are selected.) When selection is made down to the lowerside of the screen, pixel row 51 b to be written disappears, thoughpixel row 51 a to be written is present. That is, only one pixel row isleft for selection. For this reason, the current applied to sourcesignal line 18 is wholly written to pixel row 51 a. Accordingly, pixelrow 51 to be written now is undesirably programmed with a current twiceas high as the current with which the preceding pixel rows 51 a havebeen priorly programmed.

In order to solve this problem, the present invention uses a dummy pixelrow 281 formed (located) on the lower side of screen 50, as shown inFIG. 27( b). Therefore, when selection of pixel rows to be writtenreaches the lower side of screen 50, the final pixel row on screen 50and the dummy pixel row 281 are selected. For this reason, the finalpixel row shown in FIG. 27( b) is written with the regular current.Though the dummy pixel row 281 is shown to locate adjacent to the upperor lower edge of display region 50, there is no limitation to thisarrangement. The dummy pixel row 281 may be formed at a location spacedapart from display region 50. The dummy pixel row 281 need not be formedwith switching transistor 11 d, EL device 15 and the like shown inFIG. 1. The absence of these components enables the dummy pixel row 281to be reduced in size.

FIG. 28 illustrates the state shown in FIG. 27( b). As apparent fromFIG. 28, when selection of pixel rows reaches pixel 16 c on the lowerside of screen 50, the final pixel row 281 on screen 50 is selected. Thedummy pixel row 281 is located outside display region 50. That is, thedummy pixel row 281 is configured to fail to light or not to be allowedto light, or not to be seen as a display even when it lights. This canbe made by, for example, elimination of the contact hole between thepixel electrode and transistor 11 or failure to form EL film at thedummy pixel row.

Though the dummy pixel (row) 281 is provided (formed or located) on thelower side of screen 50 in the arrangement shown in FIG. 27, there is nolimitation to this arrangement. For example, in the case where scanningis performed from the lower side to the upper side of screen 50 (reversescanning) as shown in FIG. 29( a), dummy pixel row 281 should be formedalso on the upper side of screen 50, as shown in FIG. 29( b). That is,the upper side and the lower side of screen 50 are formed (provided)with respective dummy pixel rows 281. Such an arrangement canaccommodate to vertical reversal of scanning over the screen.

The above-described embodiment is configured to select two pixel rows ata time. However, the present invention is not limited to thisconfiguration but may employ a configuration for selection of, forexample, five pixel rows at a time (see FIG. 23.) That is, where fivepixel rows are driven at a time, four dummy pixel rows 281 should beformed. The dummy pixel row configuration or the dummy pixel row drivingmethod according to the present invention is of the type using at leastone dummy pixel row. Of course, it is preferable to combine the dummypixel row driving method with the N-fold pulse driving method.

With the driving method in which plural pixel rows are selected at atime, it becomes more difficult to accommodate variations in thecharacteristics of transistors 11 a as the number of pixel rows to beselected at a time increases. However, with increasing number of pixelrows to be selected, the programming current for each pixel becomeshigher and, hence, a higher current is to be passed through EL device15. If the current passing through EL device 15 is high, EL device 15 iseasy to deteriorate.

The method illustrated in FIG. 30 is capable of solving this problem.The basic concept of the method illustrated in FIG. 30 according to thepresent invention is a combination of a method such as to select pluralpixel rows at a time in a ½H period (½ of a horizontal scanning period),similarly to the methods described in relation to FIGS. 22 and 29, and amethod such as to select one pixel row in the subsequent ½H period (½ ofa horizontal scanning period), similarly to the methods described inrelation to FIGS. 5 and 13. Such a combination accommodates variationsin the characteristics of transistors 11 a and hence is capable ofmaking the responsiveness high and the in-plane uniformity satisfactory.

For easy explanation, description will be made of such a combined methodincluding selecting five pixel rows at a time in a first period and thenselecting one pixel row in a second period. In the first period (thefirst ½H), five pixel rows are selected at a time as shown in FIG. 30( a1). Since this operation has already been described with reference toFIG. 22, description thereof will be omitted. The current to be passedthrough source signal line 18 is, for example, 25 times as high as thepredetermined value. Accordingly, the transistor 11 a of each pixel (inthe case of the pixel configuration shown in FIG. 1) is to be programmedwith a 5-fold current (25/5 pixel rows=5.) Since source signal line 18is to be fed with a 25-fold current, the parasitic capacity occurring insource signal line 18 and the like can be charged/discharged in a veryshort time. Therefore, the potential of source signal line 18 becomes atarget potential in a short time and the capacitor 19 of each pixel 16is programmed to have such a terminal voltage as to pass the five-foldcurrent. The period for which the 25-fold current is applied is thefirst ½H (½ of one horizontal scanning period.)

As a matter of course, since five pixel rows are to be written with thesame image data, the transistors 11 d of these five pixel rows are madeto assume off-state so that the five pixel rows do not display. Thus,the resulting display state is as shown in FIG. 30( a 2).

In the latter ½H period, one pixel row is selected and current-based(voltage-based) programming is performed. This state is illustrated inFIG. 30( b 1). The pixel row 51 a written is programmed with a current(voltage) so that a 5-fold current will pass as in the first period. Thecurrent to be passed through each pixel in the case of FIG. 30( a 1) andthat in the case of FIG. 30( b 1) are equalized to each other because avariation in the terminal voltage of capacitor 19 is reduced to allow acurrent of a target value to pass more promptly.

Specifically, in the operation illustrated in FIG. 30( a 1), pluralpixels are fed with a current so that the terminal voltage of eachcapacitor 19 can rapidly reach a value causing an approximate current topass. At this first step, programming is made at plural transistors 11 aand, hence, errors in regard to a target value occur due to variationsin the characteristics of the transistors. At the subsequent secondstep, only the pixel row to be written with data and hold the data isselected so that programming is completed with a current having thepredetermined target value varied from the approximate target value.

Since the operation of scanning non-lighting region 52 as well as pixelrow 51 a to be written downwardly of the screen is the same as in thecase of FIG. 13 or the like, description thereof will be omitted.

FIG. 31 shows driving waveforms for realizing the driving methodillustrated in FIG. 30. As can be seen from FIG. 31, a 1H period (onehorizontal scanning period) comprises two phases. Switching betweenthese two phases is made using ISEL signal, which is shown in FIG. 31.

Reference is first made to such ISEL signal. The driver circuit 14 forcarrying out the method illustrated in FIG. 30 has first and secondcurrent output circuits. These first and second current output circuitseach comprise a DA circuit for DA conversion of 8-bit gray scale data,an operational amplifier, and the like. In the embodiment of FIG. 30,the first current output circuit is configured to output a 25-foldcurrent, while the second current output circuit configured to output a5-fold current. Outputs of the respective first and second currentoutput circuits are applied to source signal line 18 by control over aswitching circuit formed (located) in a current output section with theISEL signal. Each source signal line is provided with the first andsecond current output circuits.

When the ISEL signal assumes an L level, the first current outputcircuit adapted to output a 25-fold current is selected so that sourcedriver 14 absorbs the current from source signal line 18 (more exactly,the first current output circuit formed in source driver 14 absorbs thecurrent.) The magnitude of the current to be outputted from each of thefirst and second current output circuits can be adjusted to a 25-foldvalue, 5-fold value or the like easily, because each current outputcircuit can be formed using plural resistors and an analog switch.

When the pixel row to be written is the first pixel row (see the columnof 1H in FIG. 30) as shown in FIG. 30, gate signal lines 17 a(1) to 17a(5) are in the selected state (in the case of the pixel configurationshown in FIG. 1.) That is, the switching transistors 11 b andtransistors 11 c of pixels rows (1) to (5) are in on-state. Since theISEL is assuming the L level, the first current output circuit foroutputting a 25-fold current is selected and connected to source signalline 18. Further, gate signal line 17 b is under application ofoff-voltage (Vgh). Accordingly, the switching transistors 11 d of thepixel rows (1) to (5) are in off-state, thus preventing current frompassing through the EL devices 15 of the respective pixel rows. That is,these EL devices are in the non-lighting state 52.

Ideally, the transistors 11 a of five pixels each pass a current of Iw×2through source signal line 18. Then, the capacitor 19 of each pixel 16is programmed with a 5-fold current. Here, description is made on theassumption that transistors 11 a are uniform in characteristics (Vt andS value) for easy understanding.

Since the number of pixel rows selected at a time is five (K=5), fivedriving transistors 11 a operate. That is, a 5-fold (25/5=5) currentpasses through transistor 11 a per pixel. Source signal line 18 is fedwith a current as the sum of programming currents for the fivetransistors 11 a. For example, assuming that the current to be passedthrough pixel row 51 a to be written is Iw according to the conventionaldriving method, a current of Iw×25 is passed through source signal line18. Pixel rows 51 b to be written with image data after writing to pixelrow (1) are now used as auxiliary pixel rows for increasing the amountof current to be fed to source signal line 18. However, there arises noproblem because the pixel rows 51 b will be written with regular imagedata thereafter.

Therefore, the pixel rows 51 b each provide the same display as thepixel row 51 a during a 1H period. For this reason, at least the writtenpixel row 51 a and the pixel rows 51 b selected for increasing thecurrent are made to assume the non-lighting state 52.

In the subsequent ½H period (½ of the horizontal scanning period), onlypixel row 51 a to be written is selected. That is, only the first pixelrow is selected. As apparent from FIG. 31, only gate signal line 17 a(1)is applied with on-voltage (Vgl) while gate signal lines 17 a(2) to 17a(5) applied with off-voltage (Vgh). Therefore, the transistor 11 a ofpixel row (1) is in an operating state (the state feeding current tosource signal line 18), while the switching transistors 11 b andtransistors 11 c of the pixel rows (2) to (5) are in off-state, or inthe unselected state. Since the ISEL signal is assuming an H level, thecurrent output circuit B for outputting a 5-fold current is selected andconnected to source signal line 18. The state of gate signal line 17 bis not changed from the state assumed in the first ½H period and henceis under application of off-voltage (Vgh). Accordingly, the switchingtransistors 11 d of the pixel rows (1) to (5) are in off-state, thuspreventing current from passing through the EL devices 15 of therespective pixel rows. That is, these pixel rows are in the non-lightingstate 52.

The above-described operation causes the transistor 11 a of the pixelrow (1) to pass a current of Iw×5 through source signal line 18. Then,the capacitor 19 of each pixel row (1) is programmed with the 5-foldcurrent.

In the next horizontal scanning period, the pixel row to be written isshifted by one pixel row. That is, the pixel row to be written ischanged to pixel row (2). In the first ½H period, when the pixel row tobe written is the second pixel row as shown in FIG. 31, gate signallines 17 a(2) to 17 a(6) are in the selected state. That is, theswitching transistors 11 b and transistors 11 c of pixels rows (2) to(6) are in on-state. Since the ISEL is assuming the L level, the firstcurrent output circuit for outputting a 25-fold current is selected andconnected to source signal line 18. Further, gate signal line 17 b isunder application of off-voltage (Vgh). Accordingly, the switchingtransistors 11 d of the pixel rows (2) to (6) are in off-state, thuspreventing current from passing through the EL devices 15 of therespective pixel rows. That is, these pixel rows are in the non-lightingstate 52. On the other hand, since the gate signal line 17 b(1) of thepixel row (1) is under application of voltage Vgl, the transistor 11 dof the pixel row (1) is in on-state and the EL device 15 of the pixelrow (1) is in the lighting state.

Since the number of pixel rows selected at a time is five (K=5), fivedriving transistors 11 a operate. That is, a 5-fold (25/5=5) currentpasses through transistor 11 a per pixel. Source signal line 18 is fedwith a current as the sum of programming currents for the fivetransistors 11 a.

In the subsequent ½H period (½ of the horizontal scanning period), onlypixel row 51 a to be written is selected. That is, only the second pixelrow is selected. As apparent from FIG. 31, only gate signal line 17 a(2)is applied with on-voltage (Vgl) while gate signal lines 17 a(3) to 17a(6) applied with off-voltage (Vgh). Therefore, the transistors 11 a ofthe pixel rows (1) and (2) is in the operating state (the state wherethe pixel row (1) passes current through EL device 15 while the pixelrow (2) feeds current to source signal line 18), while the switchingtransistors 11 b and transistors 11 c of the pixel rows (3) to (6) arein off-state, or in the unselected state. Since the ISEL signal isassuming the H level, the second current output circuit for outputtingthe 5-fold current is selected. The state of gate signal line 17 b isnot changed from the state assumed in the first ½H period and hence isunder application of off-votage (Vgh). Accordingly, the switchingtransistors 11 d of the pixel rows (2) to (6) are in off-state, thuspreventing current from passing through the EL devices 15 of therespective pixel rows. That is, these pixel rows are in the non-lightingstate 52.

The above-described operation causes the transistor 11 a of the pixelrow (2) to pass a current of Iw×5 through source signal line 18. Then,the capacitor 19 of the pixel row (2) is programmed with the 5-foldcurrent. Display over one whole screen can be made by sequentiallyperforming the above-described operations.

According to the driving method described in relation to FIG. 30, Gpixel rows (G is 2 or more) are selected in the first period and each ofthe pixel rows is programmed so that an N-fold current will passtherethrough. In the second period subsequent to the first period, Bpixel rows (B is not less than 1 and less than G) are selected and eachof the pixel rows is programmed so that the N-fold current will passtherethrough.

However, another way is possible. G pixel rows (G is 2 or more) areselected in the first period and programming is made so that the totalsum of currents to pass through the respective pixel rows assumes theN-fold value. In the second period subsequent to the first period, Bpixel rows (B is not less than 1 and less than G) are selected andprogramming is made so that the total sum of currents to pass throughthe respective pixel rows assumes the N-fold value. (When one pixel rowis selected, programming is made so that the current to passtherethrough assumes the N-fold value.) For example, five pixel rows areselected at a time in FIG. 30( a 1) and a 2-fold current is passedthrough the transistor 11 a of each pixel. By so doing, source signalline 18 is fed with a 10-fold (5×2) current. In the subsequent secondperiod, one pixel row is selected in FIG. 30( b 1). The 10-fold currentis passed through transistor 11 a of this pixel row.

In the foregoing description related to FIG. 31, the period forselecting plural pixel rows at a time is set to ½H and the period forselecting one pixel row set to ½H. However, the present invention is notlimited thereto. It is possible that the period for selecting pluralpixel rows at a time is set to ¼H and the period for selecting one pixelrow set to ¾H. Further, the sum of the period for selecting plural pixelrows at a time and the period for selecting one pixel row is set to 1H.However, the present invention is not limited thereto. For example, thesum of these periods may be set to a 2H period or a 1.5H period.

In the method of FIG. 30, it is possible that the period for selectingfive pixel rows at a time is set to ½H and two pixel rows are selectedat a time in the subsequent second period. In this case also, imagedisplay without no practical trouble can be realized.

In the foregoing description related to FIG. 30, two stages are providedconsisting of the first period for selecting five pixel rows at a time,which is set to ½H, and the second period for selecting one pixel row,which is set to ½H. However, the present invention is not limitedthereto. For example, three stages may be provided consisting of thefirst period for selecting five pixel rows at a time, the second periodfor selecting two of the five pixel rows, and the third period forselecting one pixel row. That is, it is possible to write image data toa pixel row at plural stages.

The above-described N-fold pulse driving method according to the presentinvention applies the same waveform to gate signal lines 17 b ofrespective pixel rows while shifting the scanning at 1H intervals. Sucha manner of scanning makes it possible to shift a pixel row to light toanother sequentially with the lighting duration of each EL device 15 setto 1F/N. Such application of the same waveform to gate signal lines 17 bof respective pixel rows and shifting of the scanning, can be easilyrealized. This is because it is sufficient to control data ST1 and dataST2 to be applied to shift register circuits 61 a and 61 b,respectively, shown in FIG. 6. Assuming that Vgl is outputted to gatesignal line 17 b when inputted ST2 assumes L level while Vgh isoutputted to gate signal line 17 b when inputted ST2 assumes H level,ST2 to be applied to shift register 17 b is inputted at L level for a1F/N period and at H level for the rest of the period. ST2 thus inputtedshould be shifted with clock CLK2 synchronizing to 1H.

The on-off cycle of EL device 15 needs to be set to 0.5 msec or longer.If this cycle is too short, complete black display is not realized dueto human eyes having the afterimage property and, hence, the imagedisplayed is seen to blur as if the resolution is lowered. Such adisplay state is the same as the display state of a display panel of thedata holding type. On the other hand, if the on-off cycle is set to 100msec or longer, the resulting display is seen to blink. For this reason,the on-off cycle of EL device 15 has to be not less than 0.5 μsec andnot more than 100 msec, more preferably not less than 2 msec and notmore than 30 msec, much more preferably not less than 3 msec and notmore than 20 msec.

As described earlier, satisfactory motion picture display can berealized when the number by which black display screen 152 is divided(split) is one. However, flitter is likely seen on the screen.Therefore, it is preferable to split an inserted black display portioninto plural blocks. However, too much increase in the number of suchblocks results in a blurred motion picture. The number of blocksresulting from splitting has to be not less than 1 and not more than 8,preferably not less than 1 and not more than 5.

It is preferable to employ an arrangement capable of varying the numberof split blocks of a black display depending on whether a stationaryimage or a motion picture image is to be displayed. When N=4, a blackdisplay occupies 75% of the screen and an image display occupies 25% ofthe screen. In this case, when the number of split blocks is one, theblack display portion occupying 75% is scanned vertically of the screenso as to be viewed as a black band occupying 75%. When the number ofsplit blocks is 3, scanning is made so that a black display occupying25% of the screen is split into three black display blocks eachoccupying 25/3% of the screen. The number of split blocks is increasedfor stationary image display, whereas it is decreased for motion picturedisplay. Switching may be made either automatically in accordance withimages inputted (through detection of a motion picture image or thelike) or by a manual operation by the user. Alternatively, it ispossible to employ an arrangement capable of switching in accordancewith input receptacles corresponding to types of video images to bedisplayed by the display apparatus.

In a mobile phone for example, the number of split blocks is 10 or morewhen the screen is in a wallpaper display state or in an input screenstate. (In an extreme case, on/off may be made 1H by 1H. In NTSC motionpicture display, the number of split blocks is not less than 1 and notmore than 5. It is preferable to employ an arrangement capable ofchanging the number of split blocks in multiple stages, the number ofwhich is 3 or more. For example, the number of blocks is changedstepwise like 0, 2, 4, 8.

The proportion of a black display relative to the whole display screenwhich is assumed to be 1 is preferably not less than 0.2 and not morethan 0.9 (i.e., not less than 1.2 and not more than 9 in the units ofN), particularly preferably not less than 0.25 and not more than 0.6(i.e., not less than 1.25 and not more than 6 in the units of N.) If itis less than 0.20, the effect of improving motion picture display islow. If it is more than 0.9, the display portion exhibits an increasedluminance and, hence, the vertical shifting of the display portion iseasy to recognize visually.

The number of frames per second is preferably not less than 10 and notmore than 100 (i.e., not less than 10 Hz and not more than 100 Hz), morepreferably not less than 12 and not more than 65 (i.e., not less than 12Hz and not more than 65 Hz.) If the number of frames is too small,screen flitter becomes conspicuous, while if it is too large, writingfrom the driver circuit 14 or the like becomes difficult, which resultsin a degraded resolution.

Anyway, the present invention is capable of varying the brightness of animage by control over gate signal line 17. It is needless to say thatthe brightness of an image may be varied with varying current (voltage)to be applied to source signal line 18. Also, it is needless to say thatthe control over gate signal line 17 described earlier (with referenceto FIG. 33 or 35 or the like) may be combined with the art of varyingthe current (voltage) to be applied to source signal line 18.

It is needless to say that the above-described matters are applicable tothe current-based programming pixel configurations shown in FIG. 38 andthe like and the voltage-based programming pixel configurations shown inFIGS. 43, 51 and 54 and the like. It is sufficient for the transistor 11d in each of FIGS. 38, 43 and 51 to be on-off controlled. By thusturning on/off the wiring for feeding EL device 15 with current, theN-fold pulse driving method according to the present invention can berealized easily.

Application of Vgl to gate signal line 17 b for a 1F/N period may startat any time point in a 1F period (which is not limited and may be anyunit period.) This is because the purpose of such application is toobtain a predetermined mean luminance by making EL device 15 assumeon-state for a predetermined period of a unit time. However, EL device15 had better be caused to emit light by application of Vgl to gatesignal line 17 b immediately after lapse of a current-based programmingperiod (1H). This is because EL device 15 becomes less susceptible tothe influence from the current holding characteristic of capacitor 19 inFIG. 1.

It is also preferable to employ an arrangement capable of varying thenumber by which an image is to be split. For example, when the userdepresses a brightness adjustor switch or turns a brightness adjustorvolume, the value of K is varied depending on this change detected.Alternatively, it is possible to employ an arrangement such as to varythe number either manually or automatically in accordance with theparticulars of or data on an image to be displayed.

Such an arrangement for varying the value of K (i.e., the number bywhich image display portion 53 is to be split) can be realized easily.This is because it is sufficient to provide an arrangement capable ofcontrolling or varying the timing at which data is applied to ST in FIG.6 (i.e., the timing at which ST is made to assume L level in a 1Fperiod.)

While description in relation to FIG. 16 and the like has been made ofthe feature that a (1F/N) period for which gate signal line 17 b isapplied with Vgl is divided into plural subperiods (the number ofsubperiods is K) and gate signal line 17 b is applied with Vgl for a1F/(K/N) period K times, there is no limitation to this feature. Gatesignal line 17 b may be applied with Vgl for a 1F/(K/N) period L times(L≠K). That is, the present invention has the feature that image 50 isdisplayed by controlling the period (time) for which EL device 15 is fedwith current. Therefore, the art of repeating the 1F/(K/N) period Ltimes (L≠K) is included in the technical concept of the presentinvention. The luminance of image 50 can be varied digitally with avariation in the value of L. For example, the difference between thecase of L=2 and the case of L=3 corresponds to a 50% change in luminance(contrast). It is needless to say that these controls are applicable toother embodiments of the present invention. (Of course, they areapplicable to embodiments of the present invention to be describedhereinafter.) Such controls are included in the scope of the N-foldpulse driving method according to the present invention.

The foregoing embodiments are each configured to cause the display ofscreen 50 to be turned on/off by controlling transistor 11 d serving asa switching device located (or formed) between EL device 15 and drivingtransistor 11 a. This driving method solves the problem of insufficientwriting with current in a black display state of a current-basedprogramming configuration, thereby realizing a satisfactory resolutionor black display. That is, the current-based programming is highlyadvantageous in that a satisfactory black display can be realized. Thedriving method to be described next is a method capable of realizing asatisfactory black display by resetting driving transistor 11 a.Hereinafter, this embodiment will be described with reference to FIG.32.

The pixel configuration shown in FIG. 32 is basically the same as thatshown is FIG. 1. In the pixel configuration shown in FIG. 32, current Iwas programmed is passed through EL device 15 to cause EL device 15 toemit light. That is, driving transistor 11 a becomes capable of holdingthe ability to pass the current when programmed. The driving methodapplied to the FIG. 32 configuration is a method which utilizes theability to pass current to reset (or turn off) transistor 11 a.Hereinafter, this type of driving will be referred to as “resetdriving”.

In order to realize the reset driving with the pixel configuration ofFIG. 1, an arrangement capable of on-off controlling transistors 11 band 11 c independently of each other is needed. Specifically, such anarrangement is capable of controlling gate signal line 17 a (gate signalline WR) for on-off controlling transistor 11 b and signal line 17 c(gate signal line EL) for on-off controlling transistor 11 c,independently of each other. Controls over gate signal lines 17 a and 17c can be achieved using two independent shift registers 61 as shown inFIG. 6.

The driving voltage for gate signal line WR and that for gate signalline EL preferably are made different from each other. The amplitude ofthe driving voltage for gate signal line WR (the difference betweenon-voltage and off-voltage) is made smaller than that of the drivingvoltage for gate signal line EL. Basically, if the amplitude of thedriving voltage for a gate signal line is large, a punch-through voltageacross the gate signal line and the pixel becomes high, which causesunclear black to occur. The amplitude of the driving voltage for a gatesignal line WR can be adjusted by controlling the potential of sourcesignal line 18 not to be applied (or to be applied in the selectedstate) to pixel 16. Since fluctuations in the potential of source signalline 18 are small, the amplitude of the driving voltage for gate signalline WR can be decreased. On the other hand, gate signal line EL isrequired to on-off control the EL device. Therefore, the amplitude ofthe driving voltage for gate signal line EL is large. As a measure todeal with this inconvenience, the output voltages of the respectiveshift registers 61 a and 61 b are made different from each other. In thecase where each pixel comprises p-channel transistors, the off-voltagesVgh of the respective shift registers 61 a and 61 b are substantiallyequalized to each other, while the on-voltage Vgl of shift register 61 ais made lower than that of shift register 61 b.

Hereinafter, the reset driving method will be described with referenceto FIG. 33. FIG. 33 is an explanatory diagram illustrating the principleof the reset driving method. First, as shown in FIG. 33( a), transistors11 c and 11 d are turned off, while transistor 11 b turned on. Then, thedrain terminal (D) and the gate terminal (G) of driving transistor 11 aare shortcircuited, thus allowing current Ib to pass therethrough.Generally, transistor 11 a has been programmed with current in theimmediately preceding field (frame) and hence has the ability to passcurrent. When transistors 11 d and 11 b assume off-state and on-state,respectively, with transistor 11 a in that condition, driving current Ibis passed to the gate terminal (G) of transistor 11 a, so that thepotential at the gate terminal (G) and that at the drain terminal (D)are equalized to each other, thus resetting transistor 11 a(to a statenot allowing current to pass therethrough).

The reset state (the state not allowing current to pass) of transistor11 a is equivalent to an offset voltage holding state of a voltageoffset canceller configuration, which will be described later withreference to FIG. 51 and the like. That is, in the state shown in FIG.33( a), an offset voltage is held across the terminals of capacitor 19.This offset voltage has a voltage value which varies with variations inthe characteristics of transistor 11 a. Therefore, when the operationillustrated in FIG. 33( a) is performed, transistor 11 a does not passcurrent to capacitor 19 of each pixel 19. (That is, a black displaycurrent (substantially equal to zero) is held.)

It is preferable to perform an operation of turning transistors 11 b and11 c off and transistor 11 d on to pass the driving current throughdriving transistor 11 a prior to the operation illustrated in FIG. 33(a). Preferably, this operation is completed in a very short time. Thisis because current might pass through EL device 15 to cause it to lightthereby causing the display contrast to lower. The time period for thisoperation is preferably not less than 0.1% and not more than 10% of a 1Hperiod (one horizontal scanning period), more preferably not less than0.2% and not more than 2% of a 1H period. Stated otherwise, the timeperiod is preferably not less than 0.2 μsec and not more than 5 μsec.The aforementioned operation (the operation to be performed before theoperation of FIG. 33( a)) may be performed on all the pixels 16 presentin the whole screen collectively. The operations described above cancause the drain terminal (D) voltage of driving transistor 11 a to lowerthereby allowing current Ib to pass smoothly in the state shown in FIG.33( a). The above-described matters are applicable to other resetdriving methods of the present invention.

As the state shown in FIG. 33( a) continues for a longer time, theterminal voltage of capacitor 19 tends to become lower due to passage ofcurrent Ib. Therefore, the time period for which the state shown in FIG.33( a) continues needs to be fixed. According to the experiment andstudy conducted by the inventors et al, the time period for which thestate shown in FIG. 33( a) continues is preferably not less than 1H andnot more than 5H. Preferably, this period is varied depending on R, Gand B pixels. This is because these different color pixels employdifferent EL materials, which are different in threshold voltage and thelike from each other. The optimum periods for the respective R, G and Bpixels are established depending on the respective EL materials. Thoughthis period is set not less than 1H and not more than 5H in thisembodiment, it is needless to say that the period may be set to 5H ormore in a driving method based mainly on insertion of a black display(writing of a black display to the screen.) It should be noted that theblack display state of each pixel becomes better as this period becomeslonger.

After the state shown in FIG. 33( a) continued for the time period notless than 1H and not more than 5H, the pixel configuration is turnedinto the state shown in FIG. 33( b). In the state shown in FIG. 33( b),transistors 11 c and 11 b are in on-state, while transistor 11 d inoff-state. As described earlier, the state shown in FIG. 33( b) is astate where current-based programming is being performed. That is,source driver 14 outputs (or absorbs) programming current Iw to drivingtransistor 11 a. Driving transistor 11 a is programmed to have such agate terminal (G) potential as to cause current Iw to pass. (Thepotential thus set is held in capacitor 19.)

If the programming current Iw is 0 (A), transistor 11 a is kept in thestate shown in FIG. 33( a) which does not allow current to pass, thusrealizing a satisfactory black display. In the case of current-basedprogramming for a white display by the state shown in FIG. 33( b),perfect current-based programming can be achieved from the offsetvoltage providing a black display even when there are variations in thecharacteristics of driving transistors of pixels. Therefore, the timesrequired for respective driving transistors to be programmed with atarget value are equalized to each other for each gray level. For thisreason, there occurs no gray scale error due to variations in thecharacteristics of transistors 11 a and, hence, satisfactory imagedisplay can be realized.

After the current-based programming in the state shown in FIG. 33( b),transistors 11 b and 11 c are turned off and transistor 11 d turned onto cause driving transistor 11 a to pass programming current Iw(=Ie)through EL device 15, thereby causing EL device 15 to emit light.Description of the details of the state shown in FIG. 33( c) will beomitted since similar description has bee made earlier with reference toFIG. 1 and the like.

The driving method (reset driving) illustrated in FIG. 33 comprises: afirst operation in which driving transistor 11 a and EL device 15 aredisconnected from each other (or turned into a state preventing currentfrom passing therebetween), while the drain terminal (D) and the gateterminal (G) of driving transistor 11 a (alternatively, the sourceterminal (S) and the gate terminal (G) of driving transistor 11 a; inmore general term, two terminals of driving transistor 11 a includingthe gate terminal (G)) are shortcircuited; and a second operation inwhich driving transistor 11 a is programmed with current (voltage) afterthe first operation. It is at least required that the second operationbe performed after the first operation. For the reset driving to beeffected, it is necessary to provide an arrangement capable ofcontrolling transistors 11 b and 11 c independently of each other asshown in FIG. 32.

The image display state changes as follows (provided instantaneouschanges can be observed.) First, a pixel row to be programmed withcurrent is turned into a reset state (i.e., black display state). Afterlapse of 1H, current-based programming is performed. (At this time,image display is still in the black display state because transistor 11d is in off-state.) Subsequently, each EL device 15 is fed with current,so that the pixel row emits light at a predetermined luminance (with acurrent as programmed). Specifically, it should be seen that the pixelrow displaying black moves downwardly of the screen and the imagedisplayed is rewritten at a position that the pixel row has just passed.Though the current-based programming is performed 1H after the resettingaccording to the above description, the period between the programmingand the resetting may be about 5H or less. This is because a relativelylong time is required for the resetting operation shown in FIG. 33( a)to be completed. If this period is set to 5H, five pixel rows willdisplay black. (If the pixel row programmed with current is taken intoaccount, six pixel rows will display black.)

There is no limitation to the feature that resetting is made pixel rowby pixel row, but a set of plural pixel rows may be reset at a time;that is, resetting may made plural pixel rows by plural pixel rows.Alternatively, it is possible to perform resetting plural pixel rows byplural pixel rows while performing overlapped scanning. For example, iffour pixel rows are to be reset at a time, an exemplary manner ofdriving is as follows: pixel rows (1) to (4) are reset in the firsthorizontal scanning period (one unit); subsequently, pixel rows (3) to(6) reset in the second horizontal scanning period; subsequently, pixelrows (5) to (8) reset in the third horizontal scanning period; and then,pixel rows (7) to (10) reset in the fourth horizontal scanning period.Of course, the driving operations shown in FIGS. 33( b) and 33(c) areperformed in synchronism with the driving operation shown in FIG. 33(a).

It is needless to say that the driving operations shown in FIGS. 33( b)and 33(c) may be performed after all of the pixels present in one screenhave been reset either at a time or in a scanned fashion. It is alsoneedless to say that interlaced driving (scanning every other pixel rowor every other set of plural pixel rows) may be effected to reset everyother pixel row or every other set of plural pixel rows. Randomresetting is also possible. The reset driving according to the presentinvention described above is a method adapted to operate pixel rows.(That is, control is made vertically of the screen.) The concept of thereset driving is not limited to the control in the direction in whichpixel rows are arranged. It is needless to say that the reset drivingmay be performed in the direction in which pixel columns are arrangedfor example.

The reset driving method illustrated in FIG. 33 can realize better imagedisplay if combined with the N-fold pulse driving method or a likemethod according to the present invention or with the interlaced drivingmethod. The method illustrated in FIG. 22, in particular, can easilyrealize an intermittent N/K-fold pulse driving method. (This is adriving method including providing plural lighting regions on onescreen. This driving method can be easily practiced if gate signal line17 b is controlled so as to turn transistor 11 d on/off. This featurehas been described earlier.) Therefore, satisfactory image display freeof flicker can be realized. This is an excellent characteristic of themethod illustrated in FIG. 22 or its variations. It is also needless tosay that the reset driving method can realize much better image displayif combined with other driving methods including, for example, thereverse bias driving method, precharge driving method and punch-throughvoltage driving method to be described later. Thus, it is needless tosay that the reset driving method can be implemented in combination withother embodiments herein described.

FIG. 34 is a diagram showing the configuration of a display apparatusfor realizing the reset driving. Gate driver 12 a controls gate signallines 17 a and 17 b of FIG. 32. Application of on-voltage andoff-voltage to gate signal line 17 a allows transistor 11 b to be on-offcontrolled. Application of on-voltage and off-voltage to gate signalline 17 b allows transistor 11 d to be on-off controlled. Gate driver 12b controls gate signal line 17 c of FIG. 32. Application of on-voltageand off-voltage to gate signal line 17 c allows transistor 11 c to beon-off controlled.

Thus, gate signal lines 17 a and 17 c are operated by gate drivers 12 aand 12 b, respectively. For this reason, it is possible to freelycontrol the timing at which transistor 11 b is turned on to resetdriving transistor 11 a and the timing at which transistor 11 c isturned on to program driving transistor 11 a with current. Referencecharacter 341 a in FIG. 34 designates the circuit of an output section.Since other features and the like are identical with or similar to thefeatures described earlier, description thereof will be omitted.

FIG. 35 is a timing chart of the reset driving. When transistor 11 a isreset by applying on-voltage to gate signal line 17 a to turn transistor11 b on, transistor 11 d is turned off by application of off-voltage togate signal line 17 b. Thus, the configuration assumes the state shownin FIG. 32( a). During this period, current Ib is passed.

According to the timing chart of FIG. 35, reset time is set to 2H(during which gate signal line is under application of on-voltage andhence transistor 11 b is in on-state.) However, there is no limitationto this feature, but the reset time may be 2H or more. In the case whereresetting can be made very rapidly, the rest time may be less than 1H.The reset time can be varied to any desired H period easily by varyingthe pulse period of DATA (ST) to be inputted to gate driver 12. Forexample, if DATA to be inputted to ST terminal assumes H level for a 2Hperiod, the reset time outputted from each gate signal line 17 a is a 2Hperiod. Similarly, if DATA to be inputted to ST terminal assumes H levelfor a 5H period, the reset time outputted from each gate signal line 17a is a 5H period.

After the reset state for a 1H period, gate signal line 17 c(1) of pixelrow (1) is applied with on-voltage. When transistor 11 c is turned on,driving transistor 11 a is written with the programming current appliedto source signal line 18 via transistor 11 c.

After the current-based programming, gate signal line 17 c of pixel row(1) is applied with off-voltage to turn transistor 11 c off, therebydisconnecting each pixel from source signal line 18. At the same time,gate signal line 17 a is also applied with off-voltage to releasedriving transistor 11 a from the reset state. (In this period, theexpression “current-based programmed state” is more proper than theexpression “reset state”.) Further, gate signal line 17 b is appliedwith on-voltage to turn transistor 11 d on, thereby causing the currentprogrammed at driving transistor 11 a to be passed through EL device 15.Since the operation on pixel row (2) and the succeeding pixel rows isthe same as that on pixel row (1) and since that operation is obviousfrom FIG. 35, description thereof will be omitted.

In FIG. 35, the reset period is a 1H period. FIG. 36 illustrates anembodiment having a reset period of 5H. The reset period can be variedto any desired H period easily by varying the pulse period of DATA (ST)to be inputted to gate driver 12. FIG. 36 is directed to the embodimenthaving settings such that DATA to be inputted to ST1 terminal of gatedriver 12 a assumes H level for a 5H period and the reset periodoutputted from each gate signal line 17 a is a 5H period. As the resetperiod becomes longer, more perfect resetting is achieved, thusrealizing better black display. However, the display luminance islowered by a degree corresponding to the proportion of the reset period.

In the embodiment of FIG. 36, the reset period is set to 5H and thereset state is continuous. However, there is no limitation to such acontinuous reset state. For example, it is possible to turn on/off thesignal outputted from each gate signal line 17 a on a 1H basis. Such anon-off operation can be easily realized by operating an enabling circuit(not shown) formed in the output section of the shift register orcontrolling the DATA (ST) pulse to be inputted to gate driver 12.

The circuit configuration shown in FIG. 34 requires at least two shiftregister circuits (one for controlling gate signal line 17 a and theother for controlling gate signal line 17 b.) For this reason, therearises a problem of gate driver 12 a having an increased circuit scale.FIG. 37 shows an embodiment wherein gate driver 12 a has a single shiftregister. The timing chart of output signals in the operation of thecircuit of FIG. 37 is as shown in FIG. 35. Attention should be given toFIGS. 35 and 37 which use different signs to designate each of gatesignal lines 17 extending from gate drivers 12 a and 12 b.

As can be clearly understood from the configuration of FIG. 37 whichadditionally includes OR circuit 371, OR is taken from the output of thecurrent stage and the output of the preceding stage of shift registercircuit 61 a and outputted to each gate signal 17 a. That is, gatesignal line 17 a outputs on-voltage for a 2H period. On the other hand,the output of shift register 61 a, as it is, is outputted to gate signalline 17 c. Therefore, gate signal line 17 c is under application ofon-voltage for a 1H period.

For example, when an H level signal is outputted to the second stage ofshift register circuit 61 a, on-voltage is outputted to gate signal line17 c of pixel 16(1), thus making pixel 16(1) programmed with current (orvoltage). At the same time, on-voltage is also outputted to gate signalline 17 a of pixel 16(2) to turn on transistor 11 b of pixel 16(2), thusresetting driving transistor 11 a of pixel 16(2).

Similarly, when an H level signal is outputted to the third stage ofshift register circuit 61 a, on-voltage is outputted to gate signal line17 c of pixel 16(2), thus making pixel 16(2) programmed with current (orvoltage). At the same time, on-voltage is also outputted to gate signalline 17 a of pixel 16(3) to turn on transistor 11 b of pixel 16(3), thusresetting driving transistor 11 a of pixel 16(3). That is, gate signalline 17 a continues to output on-voltage for a 2H period, while gatesignal line 17 c continues to be applied with on-voltage for a 1Hperiod.

Transistors 11 b and 11 c assume on-state (see FIG. 33( b)) at the sametime when each pixel is programmed (see FIG. 33( b)). For this reason,if transistor 11 c is turned into off-state prior to transistor 11 b inswitching the pixel to an unprogrammed state, transistor 11 a assumesthe reset state shown in FIG. 33( b) undesirably. To avoid thisinconvenience, transistor 11 c needs to be turned off after theturning-off of transistor 11 b . Accordingly, it is required thatcontrol be performed so that gate signal line 17 a can be applied withon-voltage prior to the application of on-voltage to gate signal line 17c.

The foregoing embodiment is applied to the pixel configuration shown inFIG. 32 (basically FIG. 1). However, the present invention is notlimited thereto. For example, this embodiment is applicable to a currentmirror pixel configuration as shown in FIG. 38. With the pixelconfiguration of FIG. 38, the N-fold pulse driving method as illustratedin FIG. 13 or 15 or the like can be practiced by on-off control overtransistor 11 e. FIG. 39 is an explanatory diagram illustrating anembodiment based on the current mirror pixel configuration shown in FIG.38. Hereinafter, a reset driving method applied to the current mirrorpixel configuration will be described with reference to FIG. 39.

As shown in FIG. 39( a), transistors 11 c and 11 e are turned off, whiletransistor 11 d turned on. Then, the drain terminal (D) and the gateterminal (G) of current-based programming transistor 11 b areshortcircuited, thus allowing current Ib to pass therethrough.Generally, transistor 11 b has been programmed with current in animmediately preceding field (frame) and hence has the ability to passcurrent. (This is natural because the gate potential is held bycapacitor 19 for a 1F period to perform image display. However, currentis not passed in the case of perfect black display.) When transistors 11e and 11 d assume off-state and on-state, respectively, with transistor11 b in that condition, driving current Ib is passed toward the gateterminal (G) of transistor 11 a. (That is, gate terminal (G) and drainterminal (D) become shortcircuited.) Accordingly, the potential at thegate terminal (G) and that at the drain terminal (D) are equalized toeach other, thus resetting transistor 11 a (to a state not allowingcurrent to pass). Since the gate terminal (G) of driving transistor 11 band that of current-based programming transistor 11 a are common,driving transistor 11 b is also reset.

Each of the reset states (the state not allowing current to pass) ofrespective transistors 11 a and 11 b is equivalent to an offset voltageholding state of the voltage offset canceller configuration, which willbe described later with reference to FIG. 51 and the like. That is, inthe state shown in FIG. 39( a), an offset voltage is held across theterminals of capacitor 19. (The offset voltage is an initiating voltagecausing current to start passing. Application of a voltage having anabsolute value equal to or larger than the absolute value of the offsetvoltage causes current to pass through transistor 11.) This offsetvoltage has a voltage value which is variable in accordance with thecharacteristics of transistors 11 a and 11 b. Therefore, when theoperation illustrated in FIG. 39( a) is performed, transistors 11 a and11 b do not pass current to capacitor 19 of each pixel. (That is, ablack display current (substantially equal to zero) state is kept;stated otherwise, resetting to the initiating voltage causing current tostart passing is made.)

As in the case of FIG. 33( a), as the reset state shown in FIG. 39( a)continues for a longer time, the terminal voltage of capacitor 19 tendsto become lower due to passage of current Ib. Therefore, the time periodfor which the state shown in FIG. 39( a) continues needs to be fixed.According to the experiment and study conducted by the inventors et al.,the time period for which the state shown in FIG. 33( a) continues ispreferably not less than 1H and not more than 10H (10 horizontalscanning periods), more preferably not less than 1H and not more than5H. Specifically, the time period is preferably not less than 20 μsecand not more than 2 msec. This holds true for the driving methodillustrated in FIG. 33.

As in the case of FIG. 33( a), when the operation is performed so thatthe reset state shown in FIG. 39( a) synchronizes to the current-basedprogrammed state shown in FIG. 39( a), the time period required for thecurrent-based programmed state shown in FIG. 39( b) to be reached fromthe reset state shown in FIG. 39( a) has a fixed value (constant value)and, therefore, there arises no problem. That is, the time period fromthe reset state shown in FIG. 33( a) or 39(a) to the current-basedprogrammed state shown in FIG. 33( b) or 39(b) is preferably not lessthan 1H and not more than 10H (10 horizontal scanning periods), morepreferably not less than 1H and not more than 5H. Specifically, the timeperiod is preferably not less than 20 μsec and not more than 2 msec. Ifthis time period is too short, driving transistor 11 is not completelyreset, while if it is too long, driving transistor 11 assumes completeoff-state, which in turn results in the current-based programming takinga longer time. In addition, the luminance of screen 50 is lowered.

Subsequently to the state shown in FIG. 39( a), the pixel configurationis turned into the state shown in FIG. 39( b) where transistors 11 c and11 b are in on-state, while transistor 11 d in off-state. The stateshown in FIG. 39( b) is a state where current-based programming is beingperformed. That is, source driver 14 outputs (or absorbs) programmingcurrent Iw and passes the programming current Iw to driving transistor11 a. Capacitor 19 is programmed with the gate terminal (G) potential ofdriving transistor 11 b so that current Iw will pass through drivingtransistor 11 a.

If the programming current Iw is 0 (A) (black display), transistor 11 bis kept in the state shown in FIG. 33( a) which does not allow currentto pass, thus realizing a favorable black display. In the case ofcurrent-based programming for white display by the state shown in FIG.39( b), perfect current-based programming can be achieved from theoffset voltage providing a black display (the initiating voltage causingthe current set in accordance with the characteristics of drivingtransistors to start passing) even when there are variations in thecharacteristics of driving transistors of respective pixels. Therefore,the times required for respective driving transistors to be programmedwith a current of a target value are equalized to each other for eachgray level. For this reason, there occurs no gray scale error due tovariations in the characteristics of transistors 11 a or 11 b and,hence, satisfactory image display can be realized.

After the current-based programming in the state shown in FIG. 39( b),transistors 11 b and 11 c are turned off and transistor 11 e turned onto cause driving transistor 11 b to pass programming current Iw(=Ie)through EL device 15, thereby causing EL device 15 to emit light.Description of the details of the state shown in FIG. 39( c) will beomitted since similar description has bee made earlier.

The driving method (reset driving) illustrated in FIG. 33 or 39comprises: a first operation in which driving transistor 11 a or 11 band EL device 15 are disconnected from each other (or turned into astate preventing current from passing therebetween by transistor 11 e or11 d), while the drain terminal (D) and the gate terminal (G) of thedriving transistor (alternatively, the source terminal (S) and the gateterminal (G) of the driving transistor, more generally, two terminals ofthe driving transistor including gate terminal (G)) are shortcircuited;and a second operation in which the driving transistor is programmedwith current (or voltage) after the first operation. It is at leastrequired that the second operation be performed after the firstoperation. The operation of disconnecting driving transistor 11 a or 11b and EL device 15 from each other is not necessarily indispensable.Even if the first operation of shortcircuiting the drain terminal (D)and the gate terminal (G) of the driving transistor is performed withoutdisconnecting driving transistor 11 a or 11 b and EL device 15 from eachother, it is possible that variations in the reset state are not soserious in some cases. Whether driving transistor 11 a or 11 b is to bedisconnected from EL device 15 or not is decided based on examination ofthe transistor characteristics of the array manufactured.

The current mirror pixel configuration shown in FIG. 39 is a drivingmethod including resetting the current-based programming transistor 11a, which results in the resetting of the driving transistor 11 b.

With the current mirror pixel configuration of FIG. 39, the operation ofdisconnecting driving transistor 11 b and EL device 15 from each otherneed not necessarily be performed in the reset state. Thus, the drivingmethod comprises: a first operation in which the drain terminal (D) andthe gate terminal (G) of the current-based programming transistor(alternatively, the source terminal (S) and the gate terminal (G) of thecurrent-based programming transistor, more generally, two terminals ofthe current-based programming transistor or the driving transistorincluding gate terminal (G)) are shortcircuited; and a second operationin which the current-based programming transistor is programmed withcurrent (or voltage) after the first operation. It is at least requiredthat the second operation be performed after the first operation.

The image display state changes as follows (provided instantaneouschanges can be observed.) First, a pixel row to be programmed withcurrent is turned into a reset state (i.e., black display state). Afterlapse of 1H, current-based programming is performed. Specifically, itshould be seen that the pixel row displaying black moves downwardly ofthe screen and the image displayed is rewritten at a position that thepixel row has just passed.

Though the foregoing description of the embodiment is directed mainly tothe current-based programming pixel configuration, the reset drivingaccording to the present invention is applicable to voltage-basedprogramming pixel configurations. FIG. 43 is an explanatory diagramillustrating a pixel configuration (panel configuration) according tothe present invention for practicing a reset driving method with avoltage-based programming pixel configuration.

In the pixel configuration shown in FIG. 43, there is formed transistor11 e for causing driving transistor 11 a to be reset. When gate signalline 17 e is applied with on-voltgage to turn transistor 11 e on, whichcauses the gate terminal (G) and the drain terminal (D) of drivingtransistor 11 a to become shortcircuited. The pixel configuration isalso formed with transistor 11 d for cutting off the current pathbetween EL device 15 and driving transistor 11 d. Hereinafter, the resetdriving method applied to the voltage-based programming pixelconfiguration will be described with reference to FIG. 44.

As shown in FIG. 44( a), transistors 11 b and 11 c are turned off, whiletransistor 11 e turned on. Then, the drain terminal (D) and the gateterminal (G) of driving transistor 11 a become shortcircuited, thusallowing current Ib to pass as shown. Accordingly, the potential at thegate terminal (G) and that at the drain terminal (D) of drivingtransistor 11 a are equalized to each other, thus resetting transistor11 a (to a state not allowing current to pass therethrough.) Before theresetting of transistor 11 a, current has been made passing throughtransistor 11 a by initially turning transistors 11 d and 11 e on andoff, respectively, in synchronism with an HD synchronizing signal, asdescribed with reference to FIG. 33 or 39. Thereafter, the operationillustrated in FIG. 44 is performed.

Each of the reset states (the state not allowing current to pass) ofrespective transistors 11 a and 11 b is equivalent to the offset voltageholding state of the voltage offset canceller configuration described inrelation to FIG. 41 or the like. That is, in the state shown in FIG. 44(a), an offset voltage (reset voltage) is held across the terminals ofcapacitor 19. This offset voltage has a voltage value which is variablein accordance with the characteristics of transistor 11 a. Therefore,when the operation illustrated in FIG. 44( a) is performed, transistor11 a does not pass current to capacitor 19 of each pixel. (That is, ablack display current (substantially equal to zero) state is kept;stated otherwise, resetting to the initiating voltage causing current tostart passing is made.)

As in the current-based programming pixel configuration, as the resetstate shown in FIG. 44( a) of the voltage-based programming pixelconfiguration continues for a longer time, the terminal voltage ofcapacitor 19 tends to become lower due to passage of current Ib.Therefore, the time period for which the state shown in FIG. 44( a)continues needs to be fixed. This time period is preferably not lessthan 0.2H and not more than 5H (five horizontal scanning periods), morepreferably not less than 0.5H and not more than 4H. Specifically, thetime period is preferably not less than 2 μsec and not more than 400μsec.

It is preferable that gate signal line 17 e and the gate signal line 17a of an antecedent pixel row form a common line. That is, gate signalline 17 e is formed as shortcircuited to gate signal line 17 a of theantecedent pixel row. This configuration is referred to as “antecedentgate control method”. The antecedent gate control method uses a waveformapplied to the gate signal line of a pixel row having been selected atleast 1H before the selection of a pixel row concerned. Therefore, theantecedent pixel row is not limited to the immediately preceding pixelrow. For example, transistor 11 a of a pixel row concerned may be resetby using the signal waveform applied to the gate signal of the pixel rownext to the immediately preceding pixel row.

More specifically, the antecedent gate control method is as follows. Itis assumed that: a pixel row concerned is the (N)th pixel row havinggate signal lines 17 e(N) and 17 a(N); a pixel row selected 1H before isthe (N−1)th pixel row having gate signal lines 17 e(N−1) and 17 a(N−1);and a pixel row to be selected 1H after the selection of the pixel rowconcerned is the (N+1)th pixel row having gate signal lines 17 e(N+1)and 17 a(N+1).

In the (N−1)th H period, when gate signal line 17 a(N−1) of the (N−1)thpixel row is applied with on-voltage, gate signal line 17 e(N) of the(N)th pixel row is also applied with on-voltage. This is because gatesignal line 17 e(N) is formed as shortcircuited to gate signal line 17a(N−1) of the antecedent pixel row. Accordingly, transistor 11 b(N−1) ofeach pixel of the (N−1)th pixel row is turned on to write the voltage ofsource signal line 18 to the gate terminal (G) of driving transistor 11a(N−1). At the same time, transistor 11 e(N) of the (N)th pixel row isturned on to shortcircuit the gate terminal (G) and the drain terminal(D) of driving transistor 11 a(N), thereby resetting driving transistor11 a(N).

In the (N)th period following the (N−1)th H period, when gate signalline 17 a(N) of the (N)th pixel row is applied with on-voltage, gatesignal line 17 e(N+1) of the (N+1)th pixel row is also applied withon-voltage. Accordingly, transistor 11 b(N) of each pixel of the (N)thpixel row is turned on to write the voltage applied to source signalline 18 to the gate terminal (G) of driving transistor 11 a(N). At thesame time, transistor 11 e(N+1) of each pixel of the (N+1)th pixel rowis turned on to shortcircuit the gate terminal (G) and the drainterminal (D) of driving transistor 11 a(N+1), thereby resetting drivingtransistor 11 a(N+1).

A similar operation proceeds for the following pixel rows. In the(N+1)th H period following the (N)th H period, when gate signal line 17a(N+1) of the (N+1)th pixel row is applied with on-voltage, gate signalline 17 e(N+2) of the (N+2)th pixel row is also applied with on-voltage.Accordingly, transistor 11 b(N+1) of each pixel of the (N+1)th pixel rowis turned on to write the voltage applied to source signal line 18 tothe gate terminal (G) of driving transistor 11 a(N+1). At the same time,transistor 11 e(N+2) of each pixel of the (N+2)th pixel row is turned onto shortcircuit the gate terminal (G) and the drain terminal (D) ofdriving transistor 11 a(N+2), thereby resetting driving transistor 11a(N+2).

With the antecedent gate control method according to the presentinvention, driving transistor 11 a is reset for a 1H period, followed byvoltage-based programming.

As in the case of FIG. 33( a), when the operation is performed so thatthe reset state shown in FIG. 44( a) synchronizes to the current-basedprogrammed state shown in FIG. 44( a), the time period required for thecurrent-based programming state shown in FIG. 44( b) to be reached has afixed value (constant value) and, therefore, there arises no problem. Ifthis time period is too short, driving transistor 11 a is not completelyreset, while if it is too long, driving transistor 11 a assumes completeoff-state, which in turn results in the current-based programming takinga longer time. Further, the luminance of screen 12 is lowered.

Subsequently to the state shown in FIG. 44( a), the pixel configurationis turned into the state shown in FIG. 44( b) where transistors 11 b isin on-state, while transistors 11 e and 11 d in off-state. The stateshown in FIG. 44( b) is a state where current-based programming is beingperformed. That is, source driver 14 outputs the programming current,which is then written to the gate terminal (G) of driving transistor 11a(i.e., capacitor 19 is programmed with the potential of the gateterminal (G) of driving transistor 11 a.) In the case of voltage-basedprogramming, transistor 11 d need not necessarily be turned off at thetime of voltage-based programming. Transistor 11 e will not be needed ifthe combination with the N-fold pulse driving method as shown in FIG. 13or 15 or the like is unnecessary or if the intermittent N/K pulsedriving method does not need to be practiced. (The intermittent NK-foldpulse driving method is a driving method including providing plurallighting regions on one screen. This driving method can be easilypracticed if transistor 11 e is caused to turn on/off.) Since thisfeature has been described earlier, description thereof will be omitted.

In the case where a white display is provided by voltage-basedprogramming using the configuration shown in FIG. 43 or the drivingmethod illustrated in FIG. 44, perfect voltage-based programming can beachieved from the offset voltage providing a black display (theinitiating voltage causing the current set in accordance with thecharacteristics of driving transistors to pass) even when there arevariations in the characteristics of driving transistors of respectivepixels. Therefore, the times required for respective driving transistorsto be programmed with a target value are equalized to each other foreach gray level. For this reason, there occurs no gray scale error dueto variations in the characteristics of transistors 11 a and, hence,satisfactory image display can be realized.

After the voltage-based programming illustrated in FIG. 44( b),transistors 11 b is turned off and transistor 11 d turned on to causedriving transistor 11 a to pass the programming current through ELdevice 15, thereby causing EL device 15 to emit light.

Thus, the reset driving method based on the voltage-based programmingillustrated in FIG. 43 comprises: a first operation in which transistor11 d is turned on and transistor 11 e turned off in synchronism with anHD synchronizing signal to pass current to transistor 11 a; a secondoperation in which driving transistor 11 a and EL device 15 aredisconnected from each other, while the drain terminal (D) and the gateterminal (G) of the driving transistor 11 a (alternatively, the sourceterminal (S) and the gate terminal (G) of the driving transistor 11 a,more generally, two terminals of the driving transistor including gateterminal (G)) are shortcircuited; and a third operation in which thedriving transistor 11 a is programmed with voltage after the secondoperation.

In the embodiment described above, transistor 11 d is on-off controlledto control the current to be passed from driving transistor 11 a (in thecase of the pixel configuration shown in FIG. 1) to EL device 15. Inorder for transistor 11 d to be on-off controlled, gate signal lines 17b need to be scanned. Such scanning requires shift register 61 (gatecircuit 12). Since shift register 61 is large in size, use of shiftregister 61 for control over gate signal lines 17 b will prevent theframe from being narrowed. The method to be described with reference toFIG. 40 solves this problem.

Though the present invention is described by reference mainly toexamples of current-based programming pixel configuration as shown inFIG. 1 and the like, the present invention is not limited to theseexamples. It is needless to say that the present invention is applicableeven to other current-based programming pixel configurations (includinga current mirror pixel configuration) as described with reference toFIG. 38 and the like. It is also needless to say that the technicalconcept of on-off control on a block-by-block basis is applicable tovoltage-based programming pixel configurations as shown in FIG. 41 andthe like. Since the present invention is directed to a method ofintermittently passing current through EL device 15, it is needless tosay that the present invention can be combined with a method ofapplication of reverse bias voltage to be described with reference toFIG. 50 or the like. Thus, the present invention can be practiced incombination with other embodiments.

FIG. 40 illustrates an embodiment of a block driving method. For easyexplanation, it is assumed that gate driver 12 is formed directly onsubstrate 71 or gate driver 12 in a silicon chip form is mounted onsubstrate 71. Further, source driver 14 and source signal lines areomitted from the figure to avoid complicated drawing.

In FIG. 40, gate signal line 17 a is connected to gate driver 12. On theother hand, gate signal line 17 b associated with each pixel isconnected to lighting control line 401. In FIG. 40, four gate signallines 17 b are connected to one lighting control line 401.

Though four gate signal lines 17 b form one block in the configuration,there is no limitation thereto but it is needless to say that one blockmay consist of more than four gate signal lines 17 b. Generally, displayregion 50 is preferably divided into 5 or more, more preferably 10 ormore, much more preferably 20 or more. If the number by which displayregion 50 is divided is too small, flicker is likely to becomeconspicuous. On the other hand, if the number is too large, the numberof lighting control lines 401 becomes large, which makes it difficult tolayout such control lines 401.

Since a QCIF display panel has 220 vertical scanning lines, these linesneed to be divided into blocks by at least 5 (i.e., 220/5 32 44),preferably 10 or more (220/10=11). There are some cases where two blocksare sufficient because less flicker occurs in display region 50 which isdivided into two blocks, one consisting of odd number rows, the otherconsisting of even number rows.

In the embodiment shown in FIG. 40, lighting control lines 401 a, 401 b,401 c, 401 d, . . . , 401 n are sequentially applied with on-voltage(Vgl) or off-voltage (Vgh) to turn EL devices 15 on/off block by block.

In the embodiment shown in FIG. 40, gate signal line 17 b and lightingcontrol line 401 do not cross each other. Therefore, the embodiment isfree from such a failure that gate signal line 17 b and lighting controlline 401 become shortcircuited. Further, since there is no capacitivecoupling between gate signal line 17 b and lighting control line 401, avery small capacitance is added when the gate signal line 17 d side isviewed from lighting control line 401. Therefore, lighting control line401 can be driven easily.

Gate driver 12 is connected to gate signal line 17 a. When gate signalline 17 a is applied with on-voltage, the pixel row associated therewithis selected and transistors 11 b and 11 c of each of the selected pixelsare turned on to program capacitor 19 of each pixel with the current(voltage) applied to source signal line 18. On the other hand, gatesignal line 17 b is connected to the gate terminal (G) of transistor 11d of each pixel. Accordingly, when lighting control line 401 is appliedwith on-voltage (Vgl), a current path is formed between drivingtransistor 11 a and EL device 15, whereas when it is applied withoff-voltage (Vgh), the anode terminal of EL device 15 is opened.

It is preferable that the control timing at which on-voltage andoff-voltage are applied to lighting control line 401 and the timing atwhich gate driver 12 outputs pixel row selecting voltage (Vgl) to gatesignal line 17 a synchronize to one horizontal scanning clock (1H).However, there is not limitation thereto.

The signal to be applied to lighting control line 401 merely on-offcontrols the current to be passed to EL device 15. That signal need notsynchronize to image data to be outputted from source driver 14. This isbecause the signal to be applied to lighting control line 401 functionsto control the current programmed at capacitor 19 of each pixel 16.Therefore, this signal need not necessarily synchronize to the pixel rowselecting signal. Even if they synchronize to each other, the clock isnot limited to 1H but may be ½H or ¼H.

In the case of the current mirror pixel configuration shown in FIG. 38,transistor 11 e can be on-off controlled if gate signal line 17 b isconnected to lighting control line 401. Thus, the block driving can berealized.

The pixel configuration shown in FIG. 32 can realize the block drivingif gate signal line 17 a is connected to lighting control signal 401 andthe reset driving is performed. In this case, the block driving methodaccording to the present invention is a driving method in which pluralpixel rows are turned into the non-lighting state (or the black displaystate) at a time using one control line.

The embodiment described above has an arrangement where one pixel rowselecting gate signal line is provided (formed) for each pixel row. Thepresent invention is not limited to this arrangement but may have suchan arrangement that one selecting gate signal line is provided (formed)for each set of plural pixel rows.

FIG. 41 illustrates an embodiment of that arrangement. For easyexplanation, the pixel configuration shown in FIG. 1 will be mainlyexemplified. In FIG. 41, gate signal line 17 a is designed to selectthree pixels (16R, 16G and 16B) at a time. The signs “R”, “G” and “B”are meant to relate to red pixel, green pixel and blue pixel,respectively.

Accordingly, selection of gate signal line 17 a causes pixels 16R, 16Gand 16B to be selected and written with data at a time. Pixel 16R writesdata from source signal line 18R to capacitor 19R, pixel 16G writes datafrom source signal line 18G to capacitor 19G, and pixel 16B writes datafrom source signal line 18B to capacitor 19B.

Transistor 11 d of pixel 16R is connected to gate signal line 17 bR.Similarly, transistor 11 d of pixel 16G is connected to gate signal line17 bG, while transistor 11 d of pixel 16B is connected to gate signalline 17 bB. Accordingly, EL device 15R of pixel 16R, EL device 15G ofpixel 16G and EL device 15B of pixel 16B can be on-off controlledindependently of each other. That is, EL device 15R, EL device 15G andEL device 15B can be individually controlled as to their lighting timeand lighting cycle by individual control over gate signal lines 17 bR,17 bG and 17 bB.

In realizing this operation, it is suitable that the configuration shownin FIG. 6 is formed (provided) with the four shift register circuits:shift register circuit 61 for scanning gate signal line 17 a, shiftregister circuit 61 for scanning gate signal line 17 bR, shift registercircuit 61 for scanning gate signal line 17 bG, and shift registercircuit 61 for scanning gate signal line 17 bB.

In spite of the foregoing description of the feature that a current Ntimes as high as the predetermined current is passed through sourcesignal line 18 to feed EL device 15 with the current N times as high asthe predetermined current for a 1/N period, this feature cannot berealized practically. This is because actually the signal pulse appliedto gate signal line 17 punches through capacitor 19 thereby making itimpossible to set a desired voltage value (or current value) atcapacitor 19. Generally, a voltage value (or current value) lower than adesired voltage value (or current value) is set at capacitor 19. Forexample, even when driving is performed so as to set a 10-fold currentvalue, a current having about 5-fold value at most can be set atcapacitor 19. Even when N=10, EL device 15 is actually fed with acurrent equal to the current that is fed thereto when N=5. Thus, thepresent invention is directed to a driving method including setting anN-fold current value so that EL device can be fed with a current that isproportional to or corresponding to the N-fold value, or a drivingmethod including application of a current in a pulse form having a valuehigher than a desired value to EL device 15.

The present invention is also directed to the driving method including:programming driving transistor 11 a (in the case of FIG. 1) with acurrent (or a voltage) having a value higher than a desired value (i.e.,a current such as to cause EL device 15 to exhibit a luminance higherthan a desired luminance when the current, as it is, is continuouslypassed through EL device 15); and intermittently feeding the current toEL device to cause EL device to emit light at the desired luminance.

It should be noted that a circuit compensating for the punch-throughvoltage reaching capacitor 19 is incorporated in source driver 14. Thisfeature will be described later.

It is preferable that switching transistors 11 b and 11 c of FIG. 1 eachcomprise an n-channel transistor. This is because the punch-throughvoltage reaching capacitor 19 can be lowered by such an arrangement.Further, since off-leakage at capacitor 19 is reduced, this arrangementis applicable to a low frame rate not higher than 10 Hz.

In some pixel configurations, the punch-through voltage may act toincrease the current to be fed to EL device 15. In such cases, whitepeak current increases thereby to make the contrast of image displayhigher. Thus, it is possible to realize satisfactory image display.

Conversely, such a method is effective as to improve black display byusing a p-channel transistor for each of switching transistors 11 b and11 c to allow punch through to occur. In this case, voltage Vgh is usedto turn p-channel transistor 11 b off. For this reason, the terminalvoltage of capacitor 19 slightly shifts toward the Vdd side. Thus, thegate terminal (G) voltage of transistor 11 a rises, thus leading to amore satisfactory black display. Further, since the value of current forrealizing a first-level gray scale display can be increased (i.e., agiven base current can be passed until gray level 1 is reached), theoccurrence of insufficient writing with current in current-basedprogramming can be reduced.

Other effective arrangements include an arrangement in which capacitor19 b is intentionally formed between gate signal line 17 a and the gateterminal (G) of transistor 11 a to increase punch-through voltage (seeFIG. 42( a).) This capacitor 19 b preferably has a capacitance not lessthan 1/50 and not more than 1/10 as large as the capacitance of theregularly-provided capacitor 19 a. More preferably, this value is setnot less than 1/40 and not more than 1/15 as large as the capacitance ofthe regularly-provided capacitor 19 a or not less than 1 and not morethan 10 times as large as the capacitance of the source-gate (SG) (orsource-drain (SD) or gate-drain (GD)) of transistor 11 b. Much morepreferably, the value of the capacitance is set not less than 2 and notmore than 6 times as high as the capacitance of SG. The capacitor 19 bmay be formed or located between one terminal of capacitor 19 a (or gateterminal (G) of transistor 11 a) and the source terminal (S) oftransistor 11 d. The aforementioned value of capacitance holds true forthis case.

The capacitance (Cb (pF)) of capacitor 19 b for generating punch-throughvoltage has a relationship with the capacitance (Ca (pF)) of capacitor19 a for storing charge, gate terminal (G) voltage Vw of transistor 11 aat which white peak current is passed (or at which a white rasterdisplay having the highest luminance of image display is provided), andgate terminal (G) voltage Vb at which a current for providing a blackdisplay (which current assumes a value of substantially 0 for a blackdisplay in image display) is passed. Preferably, the relationshipsatisfies the condition:Ca/(200Cb)≦|Vw−Vb|≦Ca/(8Cb)wherein |Vw−Vb| is the absolute value of the difference between aterminal voltage of the driving transistor providing a white display anda terminal voltage of the driving transistor providing a black display(that is, a varying amplitude of voltage.)

More preferably, the relationship satisfies the condition:Ca/(100Cb)≦|Vw−Vb|≦Ca/(10Cb).

Transistor 11 b should comprise a p-channel transistor which is at leastdouble-gated, more preferably triple-gated or more, much more preferablyquadruple-gated or more. It is preferable to form or locate capacitorsin parallel, each of the capacitors having a capacitance not less than 1and not more than 10 times as large as the capacitance of thesource-gate SG (or gate-drain (GD)) of transistor 11 b (in on-state.)

The feature described above is effective for not only the pixelconfiguration shown in FIG. 1 but also other pixel configurations. Forexample, in the case of a current mirror pixel configuration as shown inFIG. 42( b), a capacitor for causing punch through is located or formedbetween gate signal line 17 a or 17 b and the gate terminal (G) oftransistor 11 a. In this case, the n-channel of switching transistor 11c is double-gated or more. Alternatively, switching transistors 11 c and11 d each comprise a p-channel transistor which is triple-gated or more.

In the case of the voltage-based programming configuration shown in FIG.41, a capacitor 19 c for causing punch through is formed or locatedbetween gate signal line 17 c and the gate terminal (G) of drivingtransistor 11 a. Further, switching transistor 11 c is triple-gated ormore. The capacitor 19 c for causing punch through may be locatedbetween the drain terminal (D) of transistor 11 c (on the capacitor 19 bside) and gate signal line 17 a. Alternatively, the capacitor 19 c forcausing punch through may be located between the gate terminal (G) oftransistor 11 a and gate signal line 17 a. Yet alternatively, thecapacitor 19 c for causing punch through may be located between thedrain terminal (D) of transistor 11 c (on the capacitor 19 b side) andgate signal line 17 c.

A satisfactory black display can be realized by an arrangement whichsatisfies the condition:0.05 (V)≦(Vgh−Vgl)×(Cc/Ca)≦0.8 (V)wherein Ca is the capacitance of capacitor 19 a for storing charge, Ccis the source-gate capacitance of switching transistor 11 c or 11 d (Ccis the sum of the source-gate capacitance and the capacitance of acapacitor for causing punch through if the capacitor is present), Vgh isthe high-voltage signal to be applied to a gate signal line, and Vgl isthe low-voltage signal to be applied to the gate signal line.

Preferably, the condition: 0.1 (V)≦(Vgh−Vgl)×(Cc/Ca)≦0.5 (V) issatisfied.

The feature described above is also effective for the pixelconfigurations shown in FIG. 43 and the like. In the case of thevoltage-based programming pixel configuration shown in FIG. 43, acapacitor 19 b for causing punch through is formed or located betweenthe gate terminal (G) of transistor 11 a and gate signal line 17 a.

The capacitor 19 b for causing punch through is formed of source wiringand gate wiring. However, since the capacitor 19 b is formed bysuperposition of gate signal line 17 and widened source signal line oftransistor 11 on each other, the capacitor cannot be separateddistinctively from the transistor in some practical cases.

An arrangement in which switching transistors 11 b and 11 c (in the caseof the configuration shown in FIG. 1) are each formed to have a largersize than necessary as if capacitor 19 b for causing punch through isapparently formed thereby, is also included in the scope of the presentinvention. In many cases, switching transistors 11 b and 11 c are eachformed to have a channel width W/channel length ratio of 6/6 μm. Thecapacitor 19 b for causing punch through can also be formed byincreasing the ratio of W to L. For example, the W:L ratio is set notless than 2:1 and not more than 20:1, more preferably not less than 3:1and not more than 10:1.

Preferably, the capacitor 19 b for causing punch through has a magnitude(capacitance) varying depending on R, G and B modulated by pixels. Thisis because EL devices 15 for R, G and B are different from each other indriving current and in cut-off voltage. For this reason, the gateterminals (G) of respective driving transistors 11 a associated withthese EL devices 15 are programmed with different voltages (currents).For example, when the capacitor 11 bR of R pixel has a capacitance of0.02 pF, the capacitors 11 bG and 11 bB of pixels for other colors (Gpixel and B pixel) are each set to have a capacitance of 0.025 pF. Whenthe capacitor 11 bR of R pixel has a capacitance of 0.02 pF, thecapacitor 11 bG of G pixel and the capacitor 11 bB of B pixel are set tohave a capacitance of 0.03 pF and a capacitance of 0.025 pF,respectively. In this way, the offset driving current can be adjustedfor each of R, G and B by varying the capacitance of capacitor 11 bdepending on R, G and B pixels. Thus, it is possible to optimize theblack display level of each of R, G and B pixels.

While it has been described that the capacitance of the capacitor 19 bfor generating punch-through voltage is varied, the punch-throughvoltage is generated due to the relativity between the capacitance ofcapacitor 19 a for storing charge and that of capacitor 19 b forgenerating punch-through voltage. Therefore, there is no limitation tothe feature that the capacitance of capacitor 19 b is varied dependingon R, G and B pixels. The capacitance of storage capacitor 19 a may bevaried. For example, when the capacitor 11 aR of R pixel has acapacitance of 1.0 pF, the capacitor 11 aG of G pixel and the capacitor11 aB of B pixel are set to have a capacitance of 1.2 pF and acapacitance of 0.9 pF, respectively. In this case, the capacitors 19 bof the respective R, G and B pixels are set to have capacitances ofequal value. Thus, according to the present invention, at least one ofR, G and B pixels is made different from the others in the capacitanceratio between storage capacitor 19 a and capacitor 19 b for generatingpunch-through voltage. It is to be noted that both the capacitance ofstorage capacitor 19 a and that of capacitor 19 b for generatingpunch-through voltage may be varied depending on R, G and B pixels.

It is also possible to vary the capacitance of capacitor 19 b forgenerating punch-through voltage as the screen extends laterally. Sincethe gate signal rises rapidly at each pixel 16 located close to gatedriver 12 (because the through rate is high), the punch-through voltagebecomes high. At the pixel located (formed) at the end of each gatesignal line 17, on the other hand, the signal waveform becomes dulled(due to the capacitance of gate signal line 17.) This is because thepunch-through voltage becomes low due to the gate signal rising slow(because of a low through rate.) For this reason, the capacitance ofcapacitor 19 b for generating punch-through voltage is made low at eachpixel close to the connection side of gate driver 12. On the other hand,the capacitance of capacitor 19 b is made high at the end of each gatesignal line 17. For example, a variation of about 10% in the capacitanceof capacitor is provided between the right-hand extremity and theleft-hand extremity of the screen.

The punch-through voltage to be generated is determined from thecapacitance ratio between storage capacitor 19 a and capacitor 19 b forgenerating punch-through voltage. Therefore, there is no limitation tothe aforementioned feature that the capacitance of capacitor 19 b forgenerating punch-through voltage is varied as the screen extendslaterally. It is possible that the capacitance of storage capacitor 19 ais varied depending on the lateral position of capacitor 19 a on thescreen with the capacitance of capacitor 19 b for generatingpunch-through voltage being fixed in the lateral direction of thescreen. It is needless to say that both the capacitance of capacitor 19b for generating punch-through voltage and that of storage capacitor 19a may be varied as the screen extends laterally.

The N-fold pulse driving method according to the present invention has aproblem that the current to be applied to EL device 15 becomes N timesas high as in the prior art though this phenomenon is instantaneous. Insome cases, such a high current shortens the lifetime of EL device 15.Application of reverse bias voltage Vm to EL device 15 is effective insolving the problem.

In EL device 15, electrons are injected into the electron transportlayer through the cathode, while at the same time positive holesinjected into the positive hole transport layer through the anode. Theelectrons and positive holes thus injected travel to the opposite poles.At that time, they are trapped in the organic layer and carriers areaccumulated due to an energy level difference at the interface with theluminescent layer.

It is known that when space-charge is accumulated in the organic layer,molecules are oxidized or reduced to produce unstable radical anionicmolecules and radical cationic molecules, which deteriorate the filmquality thereby lowering the luminance and causing a rise in drivingvoltage during constant-current driving. An example of means to preventthis phenomenon is a modification of the device structure for reversevoltage to be applied.

When reverse bias voltage is applied, reverse current is applied, whichcauses the electrons and positive holes injected to be withdrawn towardthe cathode and the anode, respectively. Thus, the generation ofspace-charge in the organic layer is cancelled, whereby electrochemicaldeterioration of molecules can be inhibited, which ensures the EL devicehaving a prolonged lifetime.

FIG. 45 plots a variation in reverse bias voltage Vm with varyingterminal voltage of EL device 15. The “terminal voltage”, as used here,is a voltage generated when EL device 15 is fed with a rated current.The variation shown in FIG. 45, which resulted from the case where thecurrent passed through EL device 15 had a current density of 100 A/m²,had a tendency having little difference from that of the case where thecurrent passed through EL device 15 had a current density of from 50 to100 A/m². Therefore, the reverse bias voltage application method isestimated to be effective over a wide range of current density.

The ordinate represents the ratio of the terminal voltage of EL device15 resulting 2,500 hours after the starting of application of current tothe initial terminal voltage of EL device 15. Assuming, for example,that the terminal voltage resulting at the time 0 hour after thestarting of application of a current having a current density of 100A/m² is 8 (V) while the terminal voltage resulting at the time 2,500hours after the starting of application of the current having a currentdensity of 100 A/m² is 10 (V), the terminal voltage ratio is 10/8=1.25.

The abscissa represents the ratio of rated terminal voltage V0 to theproduct of reverse bias voltage Vm by time t1 for which reverse biasvoltage was applied in one cycle. For example, if the time forapplication of reverse bias voltage Vm of 60 Hz (60 Hz has no particularmeaning) is ½ (a half), t1 is equal to 0.5. Assuming that the terminalvoltage resulting at the time 0 hour after the starting of applicationof a current having a current density of 100 A/m² is 8 (V) while reversebias voltage is 8 (V), it follows that |reverse bias voltage×t1|/(ratedterminal voltage×t2)=|−8(V)×0.5|/(8(V)×0.5)=1.0.

According to FIG. 45, when |reverse bias voltage×t1|/(rated terminalvoltage×t2) is 1.0 or more, the terminal voltage ratio does not vary(that is, the terminal voltage does not vary from the initial terminalvoltage.) Application of reverse bias voltage works effectively.However, when |reverse bias voltage×t1|/(rated terminal voltage×t2) is1.75 or more, the terminal voltage ratio tends to rise. Accordingly, themagnitude of reverse bias voltage Vm and the application time ratio t1(or t2, or the ratio between t1 and t2) should be determined so that|reverse bias voltage×t1|/(rated terminal voltage×t2) may assume 1.0 ormore. Preferably, the magnitude of reverse bias voltage Vm, theapplication time ratio t1 and the like are determined so that |reversebias voltage×t1|/(rated terminal voltage×t2) may assume 1.75 or less.

Such a bias driving method requires alternate application of reversebias voltage and rated current. In the case of FIG. 46, in order toequalize the mean luminance of sample A and that of sample B per unittime, a current that instantaneously becomes higher than in the casewhere there is no application of reverse bias voltage Vm, has to bepassed in the case where there is application of reverse bias voltageVm. For this reason, the terminal voltage of EL device 15 also becomeshigher in the case where there is application of reverse bias voltage Vm(sample A of FIG. 46.)

However, even in the driving method including application of reversebias voltage, the rated terminal voltage V0 of FIG. 45 is such aterminal voltage as to satisfy the mean luminance (that is, such aterminal voltage as to cause EL device 15 to light.) (According to thespecific example mentioned herein, the rated terminal voltage V0 is aterminal voltage resulting when a current having a current density of200 A/m² is applied. Since the duty ratio is 1/2, the mean luminancethroughout one cycle is a luminance at a current density of 200 A/m².

The matter described above lies on the assumption that EL device 15 iscaused to provide a white raster display (i.e., EL device 15 is fed witha maximum current.) When the EL display apparatus displays a pictureimage, it performs gray scale display since the picture image is anatural image. Therefore, a white peak current is not constantly passedthrough EL device 15. (The white peak current is a current passing at amaximum white display. In the case of the specific example mentionedherein, the white peak current is a current having a mean currentdensity of 100 A/m².)

In the case of picture image display, in general, the current to beapplied to (passed through) each EL device 15 is about 0.2 times as highas the white peak current. (The white peak current is a current passingunder application of the rated terminal voltage. According to thespecific example mentioned herein, the white peak current is a currenthaving a current density of 100 A/m².)

Accordingly, when a picture image is displayed with the embodiment shownin FIG. 45, any value on the abscissa needs to be multiplied by 0.2.Thus, the magnitude of reverse bias voltage Vm and the application timeratio t1 (or t2, or the ratio between t1 and t2) should be determined sothat |reverse bias voltage×t1|/(rated terminal voltage×t2) may assume0.2 or more. Preferably, the magnitude of reverse bias voltage Vm, theapplication time ratio t1 and the like are determined so that |reversebias voltage×t1|/(rated terminal voltage×t2) may assume 0.35(=1.75×0.2)or less.

That is, a value of 1.0 on the abscissa (|reverse biasvoltage×t1|/(rated terminal voltage×t2) in FIG. 45 needs to be changedto 0.2. Accordingly, when the display panel displays a picture image(this state of use seems to be usual because a white raster displayseems not to be performed usually), reverse bias voltage Vm should beapplied for predetermined time t1 so that |reverse biasvoltage×t1/(rated terminal voltage×t2) may assume 0.2 or more. Even whenthe value of |reverse bias voltage×t1|/(rated terminal voltage×t2)increases, the increase in the terminal voltage ratio is not very large,as seen from FIG. 45. In view of the case where white raster display isperformed, the upper limit value of |reverse bias voltage×t1|/(ratedterminal voltage×t2) should be adjusted to 1.75 or less.

Hereinafter, the reverse bias method according to the present inventionwill be described with reference to the relevant drawings. The method ofthe present invention is based on application of reverse bias voltage Vm(or current) during a time period in which current is not passed throughEL device 15. However, there is no limitation thereto. For example, itis possible to apply reverse bias voltage Vm forcibly while current ispassing through EL device 15. This case will result in the current fedto EL device 15 stopped, hence, EL device 15 turned into thenon-lighting state (black display state.) Though the method of thepresent invention will be described focusing mainly on the feature thata current-based programming pixel configuration is applied with reversebias voltage, there is no limitation to this feature.

In a pixel configuration adapted for reverse bias driving, transistor 11g is an n-channel transistor as shown in FIG. 47. Of course, transistor11 g may be a p-channel transistor.

In FIG. 47, when gate potential control line 473 is applied with avoltage higher than the voltage applied to reverse bias line 471,transistor 11 g(N) is turned on to apply reverse bias voltage Vm to theanode of EL device 15.

In the pixel configuration of FIG. 47 or the like, gate potentialcontrol line 473 may be operated with its potential always fixed. Forexample, when voltage Vk in FIG. 47 is 0 (V), the potential of gatepotential control line 473 is fixed to 0 (V) or more (preferably 2 (V)or more). This potential is indicated at Vsg. With gate potentialcontrol line 473 in this state, when the potential of reverse bias line471 is adjusted to reverse bias voltage Vm (0 (V) or lower, preferably avoltage lower than Vk by 5 (V) or more), transistor 11 g(N) is turned onto apply reverse bias voltage Vm to the anode of EL device 15. When thevoltage of reverse bias line 471 is made higher than the voltage of gatepotential control line 473 (that is, the gate terminal (G) voltage oftransistor 11 g), transistor 11 g is turned off to stop application ofreverse bias voltage Vm to EL device 15. Of course, it is needless tosay that reverse bias line 471 may assume a high-impedance state (openstate or the like) at that time.

As shown in FIG. 48, gate driver 12 c for controlling reverse bias line471 may be formed or disposed separately. Like gate driver 12 a, gatedriver 12 c operates shiftingly in sequence, so that the position to beapplied with reverse bias voltage is shifted synchronously with thisshifting operation.

The driving method described above is capable of applying reverse biasvoltage Vm to EL device 15 by merely varying the potential of reversebias line 471 with the gate terminal (G) voltage of transistor 11 gfixed. Thus, application of reverse bias voltage Vm can be controlledeasily. Further, the driving method can lower the voltage to be appliedacross the gate terminal (G) and the source terminal (S) of transistor11 g. This holds true for the case where transistor 11 g is a p-channeltransistor.

Application of reverse bias voltage Vm is performed when EL device 15 isnot fed with current. Therefore, it is sufficient for transistor 11 g tobe turned on while transistor 11 d is off. That is, gate potentialcontrol line 473 should be applied with voltage in a manner reverse ofthe on-off logic of transistor 11 d. For example, it is sufficient forgate signal line 17 b to be connected to the gate terminals (G) ofrespective transistors 11 d and 11 g. Since transistor 11 d is of thep-channel type while transistor 11 g is of the n-channel type, theirrespective on-off operations are opposite to each other.

FIG. 49 is a timing chart of the reverse bias driving method. In thechart, an additional number such as (1) or (2) indicates the number of apixel row. For easy explanation, it is assumed that the first pixel rowis indicated at (1) and the second pixel row indicated at (2). However,there is no limitation thereto but it may be considered that (1)indicates the Nth pixel row and (2) indicates the (N+1)th pixel row.This holds true for other embodiments unless otherwise specified. Thoughthe embodiment illustrated in FIG. 49 and the like will be described byreference to the pixel configuration shown in FIG. 1 for example, thereis no limitation thereto. For example, the driving method is applicableto the pixel configurations shown in FIGS. 41, 38 and the like.

When gate signal line 17 a(1) of the first pixel row is underapplication of on-voltage (Vgl), gate signal line 17 b(1) of the firstpixel row is under application of off-voltage (Vgh). That is, transistor11 d is off and EL device 15 is not fed with current.

Reverse bias line 471(1) is applied with voltage Vsl (which causestransistor 11 g to turn on.) Accordingly, transistor 11 g is turned onto apply reverse bias voltage to EL device 15. After lapse of apredetermined time period (a time period of 1/200 or more of 1H, or 0.5μsec) from application of off-voltage (Vgh) to gate signal line 17 b,reverse bias voltage is applied. The predetermined time period (a timeperiod of 1/200 or more of 1 H, or 0.5 μsec) before application ofon-voltage (Vgl) to gate signal line 17 b, application of reverse biasvoltage is stopped. This operation is to avoid the transistors 11 d and11 g turning on at the same time.

In the next horizontal scanning period (1H), off-voltage (Vgh) isapplied to gate signal line 17 a to select the second pixel row. Thatis, on-voltage is applied to gate signal line 17 b(2). On the otherhand, on-voltage (Vgl) is applied to gate signal line 17 b to turntransistor 11 d on. Accordingly, transistor 11 a passes current throughEL device 15 to cause EL device 15 to emit light. At the same time,off-voltage (Vsh) is applied to reverse bias line 471(1) so that ELdevice 15 of the first pixel row (1) will not be applied with reversebias voltage. On the other hand, reverse bias line 471(2) of the secondpixel row is applied with voltage Vsl (reverse bias voltage).

An image displayed over one screen is rewritten by repeating thesequential operations described above. The embodiment described abovehas the feature that application of reverse bias voltage is performedduring the period in which each pixel is programmed. However, thepresent invention is not limited to the circuit configuration shown inFIG. 48. It is apparent that plural pixel rows can be consecutivelyapplied with reverse bias voltage. It is also apparent that the reversebias driving method can be combined with block driving (see FIG. 40),N-fold pulse driving, reset driving, dummy pixel driving, or a likedriving method.

There is no limitation to the feature that application of reverse biasvoltage is performed during image display. Such an arrangement ispossible that reverse bias voltage is applied for a predetermined timeperiod after the powering-off of the EL display apparatus.

Though the embodiment described above is applied to the pixelconfiguration shown in FIG. 1, it is needless to say that the embodimentis applicable to configurations adapted for application of reverse biasvoltage as shown in FIGS. 38 and 41. For example, the embodiment isapplicable to the current-based programming pixel configuration shown inFIG. 50.

FIG. 50 illustrates a current mirror pixel configuration. Transistor 11c is a pixel selecting device. When on-voltage is applied to gate signalline 17 a 1, transistor 11 c is turned on. Transistor 11 d is aswitching device having a resetting function and a function ofshortcircuiting the drain terminal (D)-gate terminal (G) of drivingtransistor 11 a. Transistor 11 d is turned on when gate signal line 17 a2 is applied with on-voltage.

Transistor 11 d is turned on 1H (one horizontal scanning period, i.e,one pixel row), preferably 3H, before the selection of the associatedpixel. If it is 3H, transistor 11 d is turned on 3H before toshortcircuit the gate terminal (G) and the drain terminal (D) oftransistor 11 a, thus turning transistor 11 a off. Accordingly,transistor 11 b is turned into a state not allowing current to passtherethough, so that EL device 15 assumes the non-lighting state.

When EL device 15 is in the non-lighting state, transistor 11 g isturned on to apply reverse bias voltage to EL device 15. Therefore, ELdevice 15 is under application of reverse bias voltage for a time periodfor which transistor 11 d is on. For this reason, transistors 11 d and11 g are turned on at the same time in terms of logic.

The gate terminal (G) voltage of transistor 11 g is fixed by applicationof voltage Vsg. When reverse bias line 471 is applied with a reversebias voltage that is sufficiently lower than Vsg, transistor 11 g isturned on.

Thereafter, when the horizontal scanning period in which an image signalis applied (written) to the pixel of concern comes, on-voltage isapplied to gate signal line 17 a 1 to turn transistor 11 c on.Accordingly, the image signal voltage outputted from source driver 14 tosource signal line 18 is applied to capacitor 19 (with transistor 11 dbeing kept in the on-state.)

When transistor 11 d is turned on, a black display is provided. As theon-time of transistor 11 d grows longer in a one-field (one frame)period, the proportion of the black display period becomes higher.Therefore, in order to adjust the means luminance throughout one field(on frame) to a desired value notwithstanding the black display periodincluded, the display luminance during a display period needs to beraised. That is, it is required that EL device 15 be fed with a highercurrent in the display period. This operation is the N-fold pulsedriving according to the present invention. Therefore, an operationcharacteristic of the present invention is to combine the N-fold pulsedriving operation and the driving operation of turning transistor 11 don to provide a black display. Also, application of reverse bias voltageto EL device 15 in the non-lighting state is a feature characteristic ofthe present invention.

The embodiment described above is of the type which includes applicationof reverse bias voltage to a pixel assuming the non-lighting state inimage display. The method of application of reverse bias voltage is notlimited to this type. If application of reverse bias voltage isperformed when an image is not displayed, it is not necessary to providereverse bias transistor 11 g for every pixel. The “non-lighting state”,as used here, means a state where reverse bias voltage is applied beforeand after use of the display panel.

In the pixel configuration of FIG. 1, for example, pixel 16 is selected(by turning transistors 11 b and 11 c on), while source driver (circuit)14 outputs voltage V0 (for example, voltage GND) as low as the sourcedriver can output and applies voltage V0 to the drain terminal (D) ofdriving transistor 11 a. With transistor 11 a in this state, turningtransistor 11 d on causes the anode of EL device 15 to be applied withvoltage V0. At the same time, the cathode Vk of EL device 15 is appliedwith voltage Vm which is lower than voltage V0 by a value from 5 to 15(V), whereby reverse bias voltage is applied to EL device 15. Transistor11 a is also turned into off-state when applied with a voltage lowerthan voltage V0 by a value from 0 to 5 (V) as voltage Vdd. By thuscausing source driver 14 to output voltage and controlling gate signalline 17, it is possible to apply reverse bias voltage to EL device 15.

The N-fold pulse driving method is capable of passing a predeterminedcurrent (a current programmed by the voltage held at capacitor 19)through EL device 15 again even after a black display has been providedonce within a one-field (one-frame) period. With the configuration ofFIG. 50, however, once transistor 11 d is turned on, capacitor 19discharges (the meaning of which includes “reduce”) electric charge heldthereat and, hence, it is not possible to feed EL device 15 with thepredetermined current (the current programmed.) Nevertheless, thecircuit of FIG. 50 has a characteristic advantage that it can operateeasily.

The embodiment described above is applied to the current-basedprogramming pixel configuration. However, the present invention is notlimited to this embodiment but may be applied to other current-basedpixel configurations as shown in FIGS. 38 and 50. The present inventionis also applicable to voltage-based programming pixel configurations asshown in FIGS. 51, 54 and 62.

FIG. 51 shows a voltage-based programming pixel configuration which issimplest in a general sense. Transistor 11 b is a selective switchingdevice, while transistor 11 a a driving transistor for feeding currentto El device 15. In this configuration, transistor (switching device) 11g for application of reverse bias voltage is located (formed) on theanode side of EL device 15.

In the pixel configuration of FIG. 51, the current to be passed throughEL device 15 is fed to source signal line 18 and then fed to the gateterminal (G) of transistor 11 a upon selection of transistor 11 b.

The basic operation of the configuration shown in FIG. 51 will bedescribed with reference to FIG. 52 for explanation of thisconfiguration. The pixel shown in FIG. 51 is of the configuration called“voltage offset canceller” and performs a four-step operation comprisingan initializing operation, a resetting operation, a programmingoperation, and light-emitting operation.

Following a horizontal synchronizing signal (HD), the initializingoperation is performed. On-voltage is applied to gate signal line 17 bto turn transistor 11 g on. Also, on-voltage is applied to gate signalline 17 a to turn transistor 11 c on. At that time, source signal line18 is applied with voltage Vdd. Accordingly, terminal a of capacitor 19b is applied with voltage Vdd. In this state, driving transistor 11 aassumes on-state to pass a feeble current through EL device 15. Thiscurrent causes the drain terminal (D) voltage of driving transistor 11 ato have an absolute value larger than at least the operating point oftransistor 11 a.

Subsequently, the resetting operation is performed. Off-voltage isapplied to gate signal line 17 b to turn transistor 11 e off. On theother hand, on-voltage is applied to gate signal line 17 c for a timeperiod T1 to turn transistor 11 b on. This time period T1 is a resettingperiod. Also, gate signal line 17 a is continuously applied withon-votlage for a 1H period. The time period T1 is preferably not lessthan 20% and not more than 90% of a 1H period. Stated otherwise, thetime period T1 is preferably not less than 20 μsec and not more than 160μsec. The ratio of the capacitance of capacitor 19 b (Cb) to that ofcapacitor 19 a (Ca), i.e., Cb:Ca, is preferably not less than 6:1 andnot more than 1:2.

In the resetting period, transistor 11 b is turned on to shortcircuitthe gate terminal (G) and the drain terminal (D) of driving transistor11 a. Accordingly, the gate terminal (G) voltage and the drain terminal(D) voltage of driving transistor 11 a become equal to each other, thusrendering transistor 11 a into an offset state (i.e., reset state: astate not allowing current to pass therethrough). The reset state is astate where the gate terminal (G) voltage of transistor 11 a assumes avalue close to the initiating voltage at which current starts passing.This gate voltage for keeping the reset state is held at terminal b ofcapacitor 19 b. Accordingly, capacitor 19 holds offset voltage(resetting voltage).

In the subsequent programming operation, off-voltage is applied to gatesignal line 17 c to turn transistor 11 b off. On the other hand, sourcesignal line 18 is applied with DATA voltage for a time period Td.Accordingly, the gate terminal (G) of driving transistor 11 a is appliedwith a voltage as the sum of DATA voltage and offset voltage (resettingvoltage.) For this reason, driving transistor 11 a becomes able to passthe current programmed.

After the programming period, off-voltage is applied to gate signal line17 a to render transistor 11 c into off-state thereby disconnectingdriving transistor 11 a from source signal line 18. Also, gate signalline 17 c is applied with off-voltage to render transistor 11 b intooff-state which is kept for a 1F period. On the other hand, gate signalline 17 b is applied with on-voltage and off-voltage periodically. Whencombined with the N-fold driving method as shown in FIG. 13 or 15 or thelike or with the interlaced driving method, this driving method canrealize better image display.

According to the driving method illustrated in FIG. 52, capacitor 19 inthe reset state holds the initiating voltage for causing current tostart passing through transistor 11 a (offset voltage or resettingvoltage). For this reason, when the gate terminal (G) of transistor 11 ais under application of the resetting voltage, the pixel is in thedarkest black display state. However, unclear black (a drop in contrast)occurs due to the coupling between source signal line 18 and pixel 16,punch-through voltage reaching to capacitor 19 or punch through attransistors. Therefore, the driving method illustrated in FIG. 52 cannotraise the display contrast.

Transistor 11 a needs to be turned off in order to apply reverse biasvoltage Vm to EL device 15. Shortcircuiting the Vdd terminal and thegate terminal (G) of transistor 11 a is sufficient to turn transistor 11a off. This feature will be described later with reference to FIG. 53.

Alternatively, voltage Vdd or a voltage for causing transistor 11 a toturn off may be applied to source signal line 18 to turn transistor 11 bon, thereby applying such a voltage to the gate terminal (G) oftransistor 11 a. This voltage turns transistor 11 a off (or into a stateallowing little current to pass therethrough (i.e., a substantiallyoff-state in which transistor 11 a has a high impedance).) Thereafter,transistor 11 g is turned on to apply reverse bias voltage to EL device15. The application of reverse bias voltage Vm may be performed on allthe pixels at a time. Specifically, source signal lines 18 are eachapplied with the voltage for causing transistor 11 a to turnsubstantially off thereby turning on transistors 11 b of all (plural)pixel rows. Accordingly, transistors 11 a are turned off. Subsequently,transistors 11 g are turned on to apply reverse bias voltage to ELdevices 15. Thereafter, the pixel rows are sequentially applied withimage signal, whereby the display apparatus displays an image.

The following description is directed to a reset driving method appliedto the pixel configuration shown in FIG. 51. FIG. 53 illustrates anembodiment of the reset driving method. As shown in FIG. 53, gate signalline 17 a connected to the gate terminal (G) of transistor 11 c of pixel16 a is also connected to the gate terminal (G) of resetting transistor11 b of pixel 16 b of the succeeding row. Similarly, gate signal line 17a connected to the gate terminal (G) of transistor 11 c of pixel 16 b isalso connected to the gate terminal (G) of resetting transistor 11 b ofpixel 16 c of the succeeding row.

Accordingly, when on-voltage is applied to gate signal line 17 aconnected to the gate terminal (G) of transistor 11 c of pixel 16 a,pixel 16 a is programmed with voltage, while at the same time theresetting transistor 11 b of pixel 16 a of the succeeding row is turnedon to reset driving transistor 11 a of pixel 16 b. Similarly, whenon-voltage is applied to gate signal line 17 a connected to the gateterminal (G) of transistor 11 c of pixel 16 b, pixel 16 b is programmedwith current, while at the same time the resetting transistor 11 b ofpixel 16 c of the succeeding row is turned on to reset drivingtransistor 11 a of pixel 16 c. In this way, reset driving based on theantecedent gate control method can be realized easily. Further, thenumber of gate signal lines routed from each pixel can be decreased.

More specific description follows. It is assumed that gate signal lines17 are applied with respective voltages as shown in FIG. 53( a); thatis, gate signal line 17 a of pixel 16 a is applied with on-voltage,while gate signal lines 17 a of other pixels 16 applied withoff-voltage. It is also assumed that gate signal lines 17 b of pixels 16a and 16 b are applied with off-voltage, while gate signal lines 17 b ofpixels 16 c and 16 d applied with on-voltage.

Under these conditions, pixel 16 a is in a state programmed with voltageand in the non-lighting state; pixel 16 b is in a reset state and in thenon-lighting state; pixel 16 c is in a state holding the programmingcurrent and in the lighting state; and pixel 16 d is in a state holdingthe programming current and in the lighting state.

After lapse of 1H, data in shift register circuit 61 of control gatedriver 12 shifts by one bit, so that the state shown in FIG. 53( b)results. Specifically, the state shown in FIG. 53( b) is such that:pixel 16 a is in a state holding the programming current and in thelighting state; pixel 16 b is in a state programmed with current and inthe non-lighting state; pixel 16 c is in a reset state and in thenon-lighting state; and pixel 16 d is in a state holding the programmingcurrent and in the lighting state.

As can be understood from the above description, the voltage applied togate signal line 17 a of each pixel of a row of concern resets drivingtransistor 11 a of each pixel of the succeeding row thereby renderingthe pixel of the succeeding row ready for voltage-based programming inthe next horizontal period. Thus, voltage-based programming is performedon pixel rows sequentially.

The antecedent gate control method can be implemented even with thevoltage-based programming pixel configuration shown in FIG. 43. FIG. 54shows an embodiment in which the pixel configuration of FIG. 43 hasconnections adapted for the antecedent gate control method.

As shown in FIG. 54, gate signal line 17 a connected to the gateterminal (G) of transistor 11 b of pixel 16 a is also connected to thegate terminal (G) of resetting transistor 11 e of pixel 16 b of thesucceeding row. Similarly, gate signal line 17 a connected to the gateterminal (G) of transistor 11 b of pixel 16 b is also connected to thegate terminal (G) of resetting transistor 11 e of pixel 16 c of thesucceeding row.

Accordingly, when on-voltage is applied to gate signal line 17 aconnected to the gate terminal (G) of transistor 11 b of pixel 16 a ,pixel 16 a becomes programmed with voltage, while at the same timeresetting transistor 11 e of pixel 16 b of the succeeding row is turnedon to reset driving transistor 11 a of pixel 16 b. Similarly, whenon-voltage is applied to gate signal line 17 a connected to the gateterminal (G) of transistor 11 b of pixel 16 b, pixel 16 b becomesprogrammed with current, while at the same time resetting transistor 11e of pixel 16 c of the succeeding row is turned on to reset drivingtransistor 11 a of pixel 16 c. In this way, reset driving based on theantecedent gate control method can be realized easily.

More specific description follows. It is assumed that gate signal lines17 are applied with respective voltages as shown in FIG. 55( a); thatis, gate signal line 17 a of pixel 16 a is applied with on-voltage,while gate signal lines 17 a of other pixels 16 applied withoff-voltage. It is also assumed that all the reverse bias transistors 11g are off.

Under these conditions, pixel 16 a is in a state programmed withvoltage; pixel 16 b is in a reset state; pixel 16 c is in a stateholding the programming current; and pixel 16 d is in a state holdingthe programming current.

After lapse of 1H, data in shift register circuit 61 of control gatedriver 12 shifts by one bit, so that the state shown in FIG. 55( b)results. Specifically, the state shown in FIG. 55( b) is such that:pixel 16 a is in a state holding the programming current; pixel 16 b isin a state programmed with current; pixel 16 c is in a reset state; andpixel 16 d is in a state holding the programming current.

As can be understood from the above description, the voltage applied togate signal line 17 a of each pixel of a row of concern resets drivingtransistor 11 a of each pixel of the succeeding row thereby renderingthe pixel of the succeeding row ready for voltage-based programming inthe next horizontal period. Thus, voltage-based programming is performedon pixel rows sequentially.

When perfect black display is performed with a current-based drivingmethod, the current programmed at the driving transistor 11 of eachpixel is 0. That is, no current is passed from source driver 14. With nocurrent, it is impossible to charge/discharge the parasitic capacityproduced in source signal line 18 as well as to vary the potential ofsource signal line 18. Accordingly, the gate potential of the drivingtransistor does not vary and, hence, capacitor 19 keeps on holding thepotential as built one frame (field) (1F) before. For example, if awhite display is given one frame before, the white display is maintainedin the next frame even when a perfect black display is desired in thenext frame.

Embodiment of Precharge Voltage Application

Hereinafter, a problem in a current driving method will mainly bedescribed, followed by a description of a constitution of the inventionrelating to a precharge voltage application which solves the problem.Note that the problem of insufficient programming can be caused not onlyin the current driving but also in the voltage driving. Therefore, theinvention is applicable also to the voltage driving. As described withreference to FIG. 1, in order to cause the light emitting element 15 ofeach of the pixels 16 to display, the transistor 11 b and 11 c arebrought into the conductive state by the gate signal line 17 a in onehorizontal scan period (1H). Then, a current Iw (programming current Iw)is drawn to the source driver 14 from the anode voltage Vdd via thetransistor 11 a and the source signal line 18. Gradation display isperformed in accordance with an amount of the current drawn to thesource driver 14. A gate voltage responsive to the drain current of thetransistor 11 a is accumulated in the condenser 19.

In addition, it is preferable to employ this embodiment in combinationwith one of other embodiments described in this specification. Forexample, a combination of this embodiment with the backward bias voltagedriving shown in FIG. 45 or 50 and a combination of this embodiment withthe driving method shown in FIG. 14, 17, 19, 24, 37, or 53 arepreferable. Also, it is needless to say that this embodiment can becombined with a panel structure. For example, this embodiment may becombined with the structure shown in FIG. 8, 9, 10, 11, 27, 40, 41, or48.

Then, the transistor 11 d is turned on by the gate signal line 17 b, andthe transistors 11 b and 11 c are turned off by the gate signal line 17a, so that a current responsive to the charge (which is the controlvoltage) in the condenser 19 is supplied from the Vdd to the lightemitting element 15 via the transistor 11 a.

The current flowing to the source signal line 18 changes graduallydepending on a product of a value of stray capacity 641 of the sourcesignal line 18 and source-drain (S-D) resistance of the transistor 11 a.Therefore, when the capacitance 641 and the resistance are increased toomuch, the current sometimes fails to reach a predetermined value in onehorizontal scan period (1H).

With a reduction in the current flowing to the source signal line 18 (inthe case of low gray scale), the source-drain (S-D) resistance of thetransistor 11 a is increased; therefore, time required for the currentto change is increased with the reduction in the current. Though itdepends on diode characteristics of the transistor 11 a and the value ofthe stray capacity 641 of the source signal line 18, it takes 50μseconds for the current flowing to the source signal line 18 to changeto 1 μA, and it takes 250μ seconds for the current to change to 10 μA,for example.

The current flowing to the source signal line 18 supplies the chargefrom the Vdd to the source signal line 18 via the transistor 11 a andchanges by changing the charge of the stray capacity 641. That is tosay, the current flowing through the transistor 11 a(the current flowingto the source signal line 18) changes with the change in voltage of thesource signal line 18. A quantity of the supplied charge is small in aregion where the current is small. The current is small in a low grayscale region (black display region). Accordingly, the change in voltageof the source signal line 18 is slowed down to slow down the change incurrent.

In order to speed up the change in current, it is necessary to apply avoltage responsive to a predetermined source current to the sourcesignal line 18. This is because a gate potential of the transistor 11 ais changed by using a time constant obtained by a product of the straycapacity and the wiring resistance of the source signal line 18.According to the above method, the transistor l ha so changes as tosupply a predetermined current to the source signal line 18.

The wiring resistance is remarkably smaller than the source-drain (S-D)resistance of the transistor 11 a. Therefore, the change due to thevoltage applied to the source signal line 18 is remarkably rapid. Forinstance, the current perfectly changes to a desired value in about 1 to3μ seconds.

However, the source voltage used for supplying the predetermined currentto the source signal line 18 changes depending on variation incurrent-voltage characteristics of the transistor 11 a. Therefore, inorder to compensate for a difference from the predetermined current, itis necessary to change the current flowing to the source signal line 18to the predetermined value by connecting to the source signal line 18 acurrent source for supplying the predetermined current.

In order to realize the above arrangement, each of output units of thesource driver 14 has a constitution shown in FIG. 63.

gray scale data (gray scale information) are transmitted via a grayscale data wiring 633 in the source driver 14. A current responsive tothe gray scale data is generated by a current generation unit (signalcurrent source) 634, and the thus-generated current is output to thesource signal line 18, so that a current responsive to the gray scale issupplied to the source signal line 18. A voltage generation unit 631generates a precharge (or discharge, in a sense of discharging a chargeof the source signal line 18) voltage. The precharge (discharge) voltagefrom the voltage generation unit 631 is so constituted as to performoutput to the source signal line 18 via a precharge switch (secondchange over switch) 636.

Since a plurality of voltage sources and a plurality of current sourcesare required after the application of the voltage responsive to the grayscale with this method of flowing the current responsive to the grayscale, the size of the circuit is increased. In turn, since one or a fewtypes of the precharge voltage is/are used in this invention, a circuitstructure of the invention is simple and the size of the circuit issmall.

The change in current is such that a speed of a change in waveform isincreased with an increase in gray scale since apparent resistance ofthe transistor 11 a is smaller in the case of high gray scale display ascompared with that in the case of low gray scale display. Accordingly, avoltage adjusted for the black display which is difficult to beprogrammed is applied, and then the predetermined current is supplied tothe source signal line 18, thereby displaying given gray scale.Alternatively, the precharge voltage is applied to the source signalline 18 only in the case of perfect black display (gray scale 0).

In addition, even in the case of applying the precharge voltage for thegray scale 0, it is preferred to vary the precharge voltage depending onR, G, and B. This is because a dark voltage of EL element 15 variesdepending on R, G, and B. Of course, the dark voltages of the ELelements 15 of R, G, and B can be set to an identical value when thedark voltages are substantially the same. Also, it is preferred tochange the precharge voltage depending on R, G, and B in the case wherea W/L ratio and a transistor size of the driving transistor 11 a varydepending on R, G, and B.

Referring to FIG. 63, the voltage corresponding to the lowest gray scale(hereinafter referred to as black voltage) is generated in the voltagegeneration unit 631, so that a current responsive to the gray scale dataof the gray scale data signal wiring 633 is output from the currentgeneration unit 634. During one horizontal scan period (1H), the voltageapplication is performed for initial 0.2 to 3μ seconds, and then acontroller (gate driver: see FIG. 1) 12 detects a duration of thehorizontal scan period so as to perform a current output, followed bysetting a conduction period of the precharge switch 636 by a clock, acounter, and the like. An output current switch (first change overswitch) 637 may continuously be in the conductive state, but it isdesirable that the output current switch 637 is in the non-conductivestate during the conduction period of the precharge switch in order toprevent the output current switch 637 from influencing on the unitcurrent sources 654 and the like. Shown in FIG. 73 is operation of theswitches 636 and 637 in one horizontal scan period.

The given black display in the low gray scale (black display region) ismore easily performed by applying the black voltage at the beginning ofthe horizontal scan period (1H). Since the high gray scale display isperformed after once performing the black display, one horizontal scanperiod might pass before the change into the high gray scale level. Inthe case of performing the high gray scale display during 2 or morehorizontal scan periods (for instance, gray scale A and gray scale B ofwhite display), the state of the source signal line changes in the orderof black, the gray scale A, black, and the gray scale B when the blackvoltage of the precharge voltage is applied in the first 1H. When theprecharge voltage is not applied to the source signal line 18, the stateof the source signal line changes from the gray scale A to the grayscale B. The variation in state of the source signal line 18 is smallerin the case of the change from the gray scale A to the gray scale B ascompared with the variation in the case of the change from the black tothe gray scale B, and the change from the gray scale A to the gray scaleB is performed more rapidly.

Accordingly, the control on the precharge switch 636 as to whether ornot the voltage generation unit 631 is applied to the source signal line18 is adapted to change depending on the gray scale to be displayed.More specifically, no voltage is applied in the high gray scale display(this control is referred to as “selected precharge” since the controlenables to apply the precharge (discharge) voltage selectively dependingon the gray scale data; in the case of performing the precharge in allgray scales, such control is referred to as “total precharge”).

Therefore, gray scale data 13 is input to the voltage output controller632 for performing the control on the precharge switch 636, so that theoutput from the voltage output controller 632 can be changed dependingon the value of the gray scale data 13.

The selected precharge in the case of performing 64 gray scale display(gray scale 0 is black and gray scale 63 is white) will be described byway of example. For instance, in a first selected precharge mode, aprecharge voltage is applied to the source signal line 18 only in thecase of the gray scale 0. Accordingly, the control method of the voltageoutput controller 632 is so decided as to output the precharge voltageof the voltage generation unit 631 to the source signal line 18 for 1 to3μ seconds during one horizontal period only in the case of the grayscale 0. In a second selected precharge mode, the precharge voltage isapplied to the source signal line 18 only in the case of the gray scales0 to 3. Accordingly, the control method of the voltage output controller632 is so decided as to output the precharge voltage of the voltagegeneration unit 631 to the source signal line 18 for 1 to 3μ secondsduring one horizontal period only in the case of the gray scales 0 to 3.It is preferred to use predetermined commands in order to change amongthe selected precharge modes and the total precharge. Also, theprecharge application period and the precharge voltage may preferably bechanged by the use of commands. Such command settings can be realizedeasily by using a command decoder circuit, an electronic volume, and thelike.

Examples of the constitution of the current generation unit are shown inFIGS. 65 to 69. Though the case of 4 bit gray scale data and 16 grayscale display is described in the following, the current generation unitcan be realized with an arbitrary number of bits. For example, thenumber of bits may be 6 (64 gray scales (260,000 colors)). Examples ofFIGS. 65 to 67 and 69 can be realized by changing the number oftransistors and the number of switches depending on a weight of thebits, and the example of FIG. 68 can be realized by changing the numberof input bits in a digital/analog converter 681.

The reference numeral 654 in FIG. 65 denotes the transistor serving asthe unit current source. Switching circuits 651 a to 651 d are connectedbetween the output 18 and the transistor (unit current source 654). Acurrent responsive to the gray scale data is output to an internalwiring 638 of the source driver 14 by changing the number of transistorsto be connected to each of the switching circuits 651 a to 651 d inaccordance with the weight of the bits of the data. The source signalline 18 is connected to the internal wiring 638. Shown in FIG. 65 is apart of the source driver for outputting current. The number of thetransistors 654 connected to the lowest bit is one; the number of thetransistors 654 connected to the upper bit next to the lowest bit istwo; the number of the transistors 654 connected to the upper bit nextto the formerly mentioned upper bit is four; and the number of thetransistors 654 connected to the uppermost bit is eight. The number ofthe transistors 654 to be connected to the output (source signal line18) changes depending on the gray scale data by turning on and off theswitch 653 in response to the gray scale data, and the current flowingto the source signal line 18 changes due to the change in the number ofthe transistors 654, thereby achieving the gray scale display.

Adjustment of gray scale width per gray scale is achieved by changingvariable resistance 656. The transistor 655 and the transistor 654 forma current mirror structure, so that a current corresponding to a mirrorratio of a current flowing through the transistor 655 flows through thetransistor 654. Because the current flowing through the transistor 655changes with the change in value of the variable resistance 656, it ispossible to change an increment in current per gray scale. The variableresistance 656 is means for changing the current, and the means forchanging the current is not limited to the variable resistance. Forexample, an electronic volume of a current output may be used as themeans for changing the current, and it is needless to say that thismodification is applicable to the variable resistance 692 of FIG. 69.

Gradation display in FIG. 66 is performed by changing the number of thetransistors 654 to be connected to the output (source signal line 18),too. However, the gray scale display shown in FIG. 66 is different fromthat shown in FIG. 65 in that a voltage of the transistor 654 fordeciding a gray scale width for one gray scale is directly controlled bya variable voltage source 661. The variable voltage source 661 is meansfor changing (adjusting) the voltage, and the means for changing thevoltage is not limited to the variable voltage source. For example, anelectronic volume of a voltage output may be used as the means forchanging the voltage.

Shown in FIG. 67 is a constitution wherein a constant current circuithaving an operation amplifier 674 and so forth is connected in place ofthe variable resistance 656 of FIG. 65. A current flowing through thetransistor 655 is decided depending on a voltage from a voltage source671 and resistance 672. A method of changing the current depending onthe gray scale is the same as that of FIGS. 65 and 66. It is preferableto use the resistance 672 as external resistance of the source driver 14because such external resistance enables to set the current flowing toeach of the unit current sources 654 at will.

Shown in FIG. 68 is a constitution wherein gray scale display isperformed by changing a current flowing to the internal wiring 638 inaccordance with a gate voltage of the transistor 683. The gate voltagechanges depending on the gray scale data. The gray scale data arechanged into analog signals by the digital/analog converter 681, and thesignals are input to the gate voltage of the transistor 683 via anoperation amplifier 682, whereby the current is changed.

The driving circuit of the EL display apparatus of the invention isrealized by using the current output circuit 635 shown in FIGS. 65 to 68generated corresponding to gray scale, the voltage generation unit 631for generating the black voltage (precharge voltage), the controller 632for controlling the precharge switch 636 and the like depending on theduration of one horizontal scan period (1H), and so forth.

The case of one output has been described in conjunction with FIGS. 65to 68 in order to simplify the description or to simplify the drawings.In the case of plural outputs, it is necessary for the currents flowingto the transistors (unit current sources) 654 of all rows to beidentical with one another in order to output an identical current foran identical gray scale in all rows.

Shown in FIG. 69 is a constitution wherein the current generation unit634 is so modified as to output an identical current in plural rows inthe constitution of FIG. 65. Referring to FIG. 69, at least one currentmirror unit is provided for a current flowing through variableresistance 692 in order to distribute the current among plural streamsusing the current mirror unit.

If necessary, another current mirror unit may be used for distributingthe current among the plural streams. An identical current is output byconnecting the gates of the transistors 654 of each row to a gate of oneof the distributed transistors 695. It is possible to distribute thecurrent with less variation in mirror ratio by disposing the transistorsforming one current mirror unit, which uses the common gate, close toeach other. A constitution downstream of the gate signal line of thetransistors 695 b and 696 c is the same as that of the transistor 695 a.

In the constitution of FIG. 66, an output from the voltage 661 issupplied to each of gates of transistors 654 in each row. Theconstitution of FIG. 66 is different from that of FIG. 65 in that anoutput current per gray scale is controlled by changing a gate voltageof each of transistors 654 through the application of the voltage fromthe voltage source 661.

Shown in FIG. 75 is a constitution in which an identical current isoutput to plural rows. With the constitution, an identical voltage isapplied to all gate signal lines of the transistors (unit currentsources) 654 of all the rows, and the voltage is supplied from thevariable voltage source 661. For example, the transistor 654 a isprovided for a first row; the transistor 654 b is provided for a secondrow, and the transistor 654 c is provided for a third row. In the casewhere threshold voltages of the transistors (unit current sources) 654vary from one another, this method may cause difference among the outputcurrents even when the outputs are for identical gray scale to causeirregularity in the form of stripes along the signal lines.

However, when the transistors are formed from a crystalline silicon, thedifference between the threshold voltages of the adjacent outputs(source signal lines 18) is small and the threshold voltage in one chipchanges gradually toward one direction; therefore, the irregularity isnot in the form of stripes during display and luminance graduallychanges from one end to the other. Thus, the constitution is free fromdeterioration in display characteristics, and the current generationunit 634 is realized by such simple constitution.

Shown in FIG. 67 is a constitution wherein a constant current source isformed by using an operation amplifier 674, a transistor 672, andresistance 673, and a current from the constant current source isadjusted in response to a mirror ratio using a transistor 655 and acurrent mirror unit to be supplied to the transistors (unit currentsources) 654. The current flowing to the unit current sources 654 isdecided depending on values of a voltage source 671, resistance 673, anda Vcc power source connected to the resistance 673.

Light emission efficiency which is one of current characteristicsrelating to luminance of an organic light emitting element variesdepending on R, G, and B in the R-G-B apposition, and, therefore, acurrent for one luminance varies as shown in FIG. 72, for example. Also,in the method using a color filter, a current for one luminance variesdepending on R, G, and B when transmittivity differs among the colors.Also, in the case of using CCM, because a color conversion efficiency offrom blue to red differs from that of from blue to green, a current forone luminance basically varies depending on the colors. Therefore, adark current also varies depending on the colors. In the example of FIG.72, the dark currents for red, green, and blue are IR, IG, and IB.

Since the voltage generated by the voltage generation unit 631 is thesource signal line voltage in the case of flowing a current required forthe lowest gray scale to the source signal line 18, the voltage variesdepending on the colors.

Accordingly, as shown in FIG. 71, voltages 711R, 711G, and 711B varyingfrom one another depending on the display colors are supplied from thevoltage generation unit 631, and a voltage corresponding to a sourcepotential at the time when a dark current of red (R) light emittingelement is flowing is supplied to 711R. Voltages corresponding to green(G) and blue (B) are supplied in the same manner to 711G and 711B.

The dark currents (Idark) are calculated from current-luminancecharacteristics of the organic light emitting element shown in FIG. 72to obtain voltages to be supplied. When one pixel has the constitutionof FIG. 1, a gate voltage of the transistor 11 a at the time when acurrent of Idark flows to the source signal line 18 is calculated fromthe current-voltage characteristics of the transistor 11 a whichcontrols the current flowing to the light emitting element 15, and thenthe thus-obtained gate voltage is generated in the voltage generationunit 631. Though the case of calculating the gate voltage of thetransistor 11 a when the current of Idark flows is described above, theinvention is not limited to the case. A current close to Idark may alsobe used for the calculation. The spirit of the invention is to generatea satisfactory precharge voltage for the black gray scale display ineach of R, G, and B circuits. Therefore, currents other than the Idarkmay be used so far as they are sufficient for practical use, and this isapplicable to the following embodiments.

Also, the pixel structure is not limited to that shown in FIG. 1, andthe invention is realized by the use of the current mirror structureshown in FIG. 70. In the current mirror structure, a gate voltage when acurrent of Idark flows through the transistor 11 b is to be generated inthe voltage generation unit 631. In short, to realize the invention, thegate voltage obtained when the transistor for controlling the currentflowing to the organic light emitting element supplies the current Idarkis generated by the voltage generation unit 631 irrelevant from thepixel circuit structure.

Further, in addition to the constitution for varying the voltagedepending on the display colors shown in FIG. 71, the output from thevoltage output controller 632 may be varied depending on the displaycolors. For example, the conduction time of the precharge switch 636 maybe varied depending on the display colors, or the gray scale for whichthe precharge switch 636 is brought into the conductive state may bevaried depending on the display colors. More specifically, the selectedprecharge may be performed only for the gray scale 0 of R withoutsetting the precharge for G and B. Alternatively, the selected prechargemay be performed for the gray scales 0 to 3 of R and the gray scale 0 ofeach of G and B. Yet alternatively, the precharge may be performed forall the gray scales of R, while performing the selected precharge forthe gray scale 0 of each of G and B.

The above output variations are proposed in view of the fact that: thetime required for a current to change to a predetermined current variesdepending on a current of each of R, G, and B; the time required for thechange is reduced with the increase in current; low gray scale displayof a display color requiring a small dark current is more easilyachieved through the application of the voltage from the voltagegeneration unit as compared with a display color requiring a large darkcurrent; and the range of the low gray scale display levels of thedisplay color requiring the small dark current is larger (closer to thehigh gray scale level) than that of the display color requiring thelarge dark current.

Particularly in the pixel structure of FIG. 64, it has been found thatthe voltage of the voltage generation unit applied for about 0.5 to 3μseconds is sufficient when the gray scale level is 0 in the case ofmanufacturing a multi-color display apparatus according to the R-G-Bapposition. Also, it has been found that the voltage application is notcharacteristics of the color of light.

For example, in the case of a multi-color display apparatus constitutedof a red light emitting element (R), a green light emitting element (G),and a blue light emitting element (B) having the luminance-currentcharacteristics shown in FIG. 72, a current for displaying black variesdepending on the colors of light, and the current for displaying blackof the green light emitting element must be smaller than that of the redlight emitting element.

In a display apparatus performing gray scale display by changing acurrent to be supplied to the pixels shown in FIGS. 64 and 70 and theorganic light emitting element owing to the change in gate potentialcaused by a current from a transistor, time required for the currentflowing through the transistor which controls the current to be suppliedto the organic light emitting element to change to a predeterminedcurrent is increased with a reduction in current. Particularly, it takesthe longest time to change to the lowest current. As a result, it isdifficult to achieve the black display since the current does not changefrom that of a preceding horizontal scan period to that of the blackdisplay in one horizontal scan period and a current indicating grayscale between the preceding gray scale and the black display issupplied.

However, when the dark current is large, it is possible to achieve theblack display even when the current flowing through the transistor isnot 0. For instance, the red light emitting element requires a currentof IR or less. It is in some cases possible to change the current to belarger than IG and equal to or smaller than IB depending on the durationof one horizontal scan period though it is impossible to change thecurrent to be equal to or smaller than IG. In this case, the red andblue pixels achieve the black display without application of voltagegenerated by the voltage generation unit 631, and only the green pixelfails to achieve the black display without the application of thevoltage.

Accordingly, as shown in FIG. 74, enable signal lines 741 for therespective display colors are input to the voltage output controller632, so that the application of the voltage of the voltage generationunit 631 is selected depending on the display colors. In the displayapparatus of the above example, an enable signal is input to 741R and741B to bring the precharge switches 636 into the non-conductive statein all horizontal scan periods irrelevant from the gray scale level,while the precharge switch 636 of 741G is closed during a partial periodin one horizontal scan period when the gray scale data 13 indicate grayscale 0. Thus, it is possible to select as to whether or not the blackvoltage is applied for each of the display colors.

Further, this method enables to reduce types of voltages to be generatedby the voltage generation unit 631 in the case of applying the voltagefor the required display color(s) as compared with the constitution ofFIG. 71. It is possible to reduce the number of voltage applicationsfrom three to one when the black voltage is applied for one color andfrom three to two when the black voltage is applied for two colors,thereby contributing to a reduction in size of the circuit of the powersource unit.

It is needless to say that the switch 636 shown in FIG. 63 can be formeddirectly on the substrate 70 by the low temperature polysilicontechnology. The same applies to the voltage generation unit 631.

It is necessary to keep the duration of the application of the prechargevoltage to be 0.5μ seconds or more. Alternatively, the precharge timemay preferably be 1% or more and 10% or less of one horizontal scanperiod. More preferably, the precharge time may be 2% or more and 8% orless of 1H.

It is preferable to change the precharge voltage depending on contents(brightness, definition, etc.) of the display image 21. For example, auser detects the change through pressing an adjustment switch or turningvolume adjustment screw to change the precharge voltage (current).Alternatively, the voltage may be automatically changed depending oncontents and data of a display image. For example, intensity of externallight is detected by a photo sensor, and the precharge (discharge)voltage (current) is adjusted by using the detected intensity.Alternatively, the voltage may be adjusted depending on types of images(an image created by using a personal computer, an image of noontime, animage of starlit sky, etc.). A degree of the adjustment is decided inview of average brightness, maximum luminance, minimum luminance,dynamic image, stationary image, and luminance distribution of theimage.

The precharge voltage may be subdivided. For example, precharge voltagesPV1, PV2, PV3, and PV4 may be generated so that: the voltage PV1 isapplied to the source signal line 18 for gray scale 0; the voltage PV2is applied to the source signal line 18 for gray scales 1 to 7; thevoltage PV4 is applied to the source signal line 18 for gray scales 8 to16; and the voltage PV4 is applied to the source signal line 18 for grayscales 59 to 63.

The precharge voltage application is not limited to the black displayregion, and, as described in the foregoing embodiments, the prechargevoltage may be applied as a white voltage to the source signal line 18to the white display region.

The precharge voltage may preferably be set to a value which isdifferent from the anode voltage Vdd (source or drain terminal voltageof the driving transistor 11 a) shown in FIG. 64 by 0.2 (V) or more and2.0 (V) or less. More preferably, the precharge voltage may be set to avalue which is different from the anode voltage Vdd by 0.4 (V) or moreand 1.2 (V) or less. For example, when the driving transistor has Pchannels and the Vdd voltage is 5.5 (V) as shown in FIG. 64, theprecharge voltage may be set to a value in the range of 5.3 (V) to 3.5(V). More preferably, the precharge voltage may be set to a value in therange of 5.1 (V) to 4.2 (V).

In addition, the current sources in general can substantially output apredetermined voltage even with a change in load impedance, and thevoltage sources in general can substantially output a predeterminedvoltage even with a change in load impedance. In turn, in thisinvention, it is necessary that at least the output impedance of thevoltage generation unit 631 for precharge voltage application is smallerthan that of the current generation unit 635 for source signal output.Of course, it is desirable that the output impedance of the currentgeneration unit 635 is sufficiently larger than the load impedance andthat the output impedance of the voltage generation unit 631 issufficiently smaller than the load impedance.

Embodiment of an Electronic Display Appliance

Next, description will be made of an embodiment of a display applianceusing the driving method of the present invention. FIG. 57 is a planview of a mobile phone as an example of a personal digital assistant.The mobile phone shown includes a receiver and a speaker. Casing 573 isprovided with an antenna 571, a numeric key pad 572 and the like. Keys572 a to 572 e include a display color switching key, a power on-off keyand a frame rate changing key.

A sequence may be formed such that depressing the display colorswitching key once will turn the display into a 8-color mode, depressingthe same key again subsequently will turn the display into a 256-colormode, and further depressing the same key will turn the display into a4096-color mode. The key is a toggle switch operative to change thedisplay color mode upon every depression. Change keys corresponding todisplay colors may be provided separately. In this case, there are three(or more) display color switching keys.

The display color switching key may be another mechanical switch, suchas a slide switch, instead of a push switch. Alternatively, it ispossible to employ an arrangement for switching the display color basedon voice recognition. Such an arrangement is possible that the displaycolor on the display screen 50 of a display panel is changed in responseto a voice inputting of, for example, “4096-color display”,“high-definition display”, “256-color mode” or “low display color mode”to the receiver. This arrangement can be realized easily by utilizingthe current voice recognition technology.

The switching of display color may be made using an electrical switch ora touch panel for the user to select a desired item from a menudisplayed in the display section 21 of the display panel by touching.Alternatively, it is possible to employ such an arrangement that thedisplay color is changed as the number of depressions on the switchvaries or as the rotation and the direction vary like a click ball.

Instead of the aforementioned display color switching key, a key forchanging the frame rate or the like may be used. A key for switchingbetween motion picture display and stationary image display may be used.It is possible to employ such an arrangement as to change pluralconditions such as the frame rates of motion picture display andstationary image display. Also, it is possible to employ such anarrangement as to gradually vary the frame rate by being continuouslydepressed. This arrangement can be realized by using a variable resistoror an electronic volume for resistor R of an oscillator comprisingcapacitor C and the resistor R, or by using a trimmer capacitor for thecapacitor C. Such an arrangement may be realized using a circuit inwhich one or more capacitors selected from plural capacitors formed on asemiconductor chip are connected in parallel.

The technical concept of varying the frame rate based on the displaycolor is applicable not only to mobile phones but also to variousapparatus of the type having a display screen such as palm-topcomputers, notebook PCs, desk-top PCs and portable clocks. This conceptis applicable not only to organic EL display panels but also to liquidcrystal display panels, transistor panels, PLZT panels, CRTs and thelike.

Though not shown in FIG. 57, the mobile phone according to the presentinvention has a CCD camera on the rear side of the casing 573. An imagetaken by this CCD camera can be immediately displayed on display screen50 of the display panel. The data on the image taken by the CCD cameracan be displayed on display screen 50. The image data taken by the CCDcamera can be displayed in different display color modes such as 24-bitmode (16,700,000 colors), 18-bit mode (260,000 colors), 16-bit mode(65,000 colors), 12-bit mode (4,096 colors), and 8-bit mode (256colors), which can be switched one from another by inputting through thekey 572.

When the display data is data of 12 bits or more, the error diffusionprocess is performed before it is display. That is, when image data fromthe CCD camera exceeds the capacity of internal memory, image processingincluding the error diffusion process and the like is performed so thatthe number of colors to be displayed will correspond to a capacity lowerthan the capacity of the internal image memory.

Now, reference is made to the case where source driver 14 is providedwith internal RAM adapted for 4,096 colors (4 bits for each of R, G andB) per screen. In the case where image data fed from outside of themodule is 4,096-color data, the data is stored directly into theinternal image RAM and then read out of the internal image RAM for theimage to be displayed on display screen 50.

In the case where image data is 260,000-color data (16-bit datacomprising 6 bits for G and 5 bits for each of R and B), the image datais temporarily stored into the operational memory of an error diffusioncontroller while, at the same time, being subjected to the errordiffusion process or dither process performed by an operational circuit.Such an error diffusion process or the like converts the 16-bit imagedata into 12-bit data, the number of bits of which is equal to that ofthe internal image RAM. The data thus converted is transferred to sourcedriver 14, which in turn outputs image data having 4 bits for each of R,G and B (4,096 colors) to display the image on display screen 50.

An embodiment employing the EL display panel or EL display apparatus orthe driving method according to the present invention will be describedwith reference to the drawings.

FIG. 58 is a sectional view of a view finder according to the embodimentof the present invention. FIG. 58 illustrates the view finderschematically for easy explanation. In this figure there are portionsenlarged or reduced in scale, or omitted. For example, an eyepiece coveris omitted from FIG. 58. This holds true for other figures.

Body 573 has a reverse surface in a dark or black color. This is forpreventing stray light emitted from EL display panel (display apparatus)574 from diffuse reflection at an internal surface of body 573. On thelight-emitting side of the display panel are located phase plate (λ/4plate or the like) 108, sheet polarizer 109 and the like. Thesecomponents have been described with reference to FIGS. 10 and 11.

Magnifying lens 582 is fitted to eyepiece ring 581. The observer adjuststhe position of the eyepiece ring 581 inserted in the body 573 so thatimage 50 displayed by the display panel 574 may be brought into focus.

If convex lens 583 is disposed on the light-emitting side of the displaypanel 574 when need arises, a principal ray incident on the magnifyinglens 582 can be converged. Therefore, it is possible to reduce thediameter of the magnifying lens 582, hence, downsize the view finder.

FIG. 59 is a perspective view of a digital video camera. The videocamera includes shooting (image pickup) lens section 592 and a digitalvideo camera body 573, the shooting lens section 592 and the view findersection 573 being positioned back to back. The view finder 573 (see FIG.58 also) is fitted with an eyepiece cover. The observer (user) observesdisplay section 50 of the display panel 574 from the eyepiece coversection.

The display section 50, which is the EL display panel of the presentinvention, is also used as a display monitor. The angle of the displaysection 50 can be adjusted about a fulcrum 591 as desired. When not inuse, the display section 50 is put in a storage section 593.

A switch 594 is a change-over switch or a control switch forimplementing the following functions. The switch 594 is a display modechange-over switch. It is preferable to provide a mobile phone or thelike with switch 594. Description will be made of this display modechange-over switch 594.

One of the driving methods according to the present invention includesfeeding EL device 15 with an N-fold current for 1/M of a 1F periodthereby causing EL device 15 to light for a 1/M period. The brightnessof EL device 15 can be varied digitally by varying this lighting period.If N=4 for example, EL device 15 is fed with a 4-fold current. If the1/M lighting period is varied by varying the value of M from 1 up to 4,the brightness can be varied from 1-fold brightness up to 4-foldbrightness. It is possible to employ an arrangement capable of varyingthe value of M in such a manner as M=1, 1.5, 2, 3, 4, 5, 6.

The above-described change-over operation is utilized for an arrangementsuch as to make display screen 50 very bright when the mobile phone ispowered on and, after lapse of a fixed time, lower the display luminanceto save the power. The change-over operation may also be utilized as afunction which allows the user to set his or her desired brightness. Forexample, when in use outdoors, the screen is made very bright, otherwisethe screen is difficult to view due to the surrounding which is brightoutdoors. However, if such a high-luminance display is continued, ELdevice will deteriorate rapidly. For this reason, if such a very brightdisplay is provided, an arrangement is employed such as to resume thenormal luminance in a short time. Further, if a high-luminance displayis needed, an arrangement is employed which allows the user to raise thedisplay luminance by his or her depressing a button.

Thus, it is preferable to employ an arrangement which allows the user tovary the brightness of the screen by button 594, an arrangement which iscapable of automatically varying the brightness of the screen accordingto preset modes, or an arrangement which is capable of detecting thebrightness of extraneous light and automatically varying the brightnessof the screen depending on the result of detection. Also, it ispreferable to employ an arrangement which allows the user or the like toset the display luminance to any value, for example, 50%, 60% or 80%.

Preferably, display screen 50 provides a Gaussian distribution display.The Gaussian distribution display is a display having a central portionmade to exhibit a higher luminance and a peripheral portion maderelatively dark. Visually, a display having a bright central portionappears to be wholly bright even when the peripheral portion is dark.According to subjective evaluation, the peripheral portion appears notto be visually inferior to the central portion as far as the peripheralportion maintains 70% of the luminance of the central portion. Not soserious a problem arises even when the luminance of the peripheralportion is further lowered to 50% of the luminance of the centralportion. In the display panel of the self-luminescence type according tothe present invention, a Gaussian distribution is provided vertically ofthe screen from the upper side to the lower side thereof by utilizingthe N-fold pulse driving method (which includes feeding EL device 15with an N-fold current for 1/M of a 1 F period.)

Specifically, the value of M is increased for the upper and lowerportions of the screen and decreased for the central portion. This canbe realized by modulating the operation speed of the shift register ofgate driver 12. The modulation of the brightness of the screen in thelateral direction is made based on multiplication of table data andimage data by each other. When the peripheral luminance is lowered to50% (with an angle of view of 0.9), the operation described above makesit possible to attain about 20% reduction in power consumption ascompared to the case of 100% display luminance. When the peripheralluminance is lowered to 70% (with an angle of view of 0.9), theabove-described operation makes it possible to attain about 15%reduction in power consumption as compared to the case of 100% displayluminance.

It is preferable to provide a change-over switch or the like for turningon/off such a Gaussian distribution display. This is because theperipheral portion of the screen giving the Gaussian distributiondisplay becomes invisible when the apparatus is used outdoors forexample. For this reason, it is preferable to employ an arrangementwhich allows the user to turn on/off the Gaussian distribution displayby a button, an arrangement which is capable of automatically switchingbetween on and off according to preset modes, or an arrangement which iscapable of detecting the brightness of extraneous light andautomatically switching between on and off depending on the result ofdetection. It is also preferable to employ an arrangement which allowsthe user to set the luminance of the peripheral portion to any value,for example, 50%, 60% or 80%.

Liquid crystal display panels, in general, use a back light to cause afixed Gaussian distribution to occur. Therefore, such a Gaussiandistribution cannot be turned on/off. The ability to turn on/off aGassian distribution is the advantage characteristic ofself-luminescence type display devices.

In the case where the frame rate is predetermined, it is possible thatflicker occurs due to interference between the panel and a lightingstate of a fluorescent lamp located indoors or the like. When EL displaydevice 15 operates at a frame rate of 60 Hz while a fluorescent lamp islighting with an alternating current of 60 Hz, there occurs slightinterference, which might make the viewer feel the screen blinkingslowly. To avoid this inconvenience, varying the frame rate issufficient. According to the present invention, the function of varyingthe frame rate is additionally provided. Further, the N-fold pulsedriving method (which includes feeding EL device 15 with an N-foldcurrent for 1/M of a 1F period) according to the present invention iscapable of varying the value of N or M.

The above-described functions can be implemented by switch 594. Whendepressed plural times, switch 594 realizes switching between theabove-described functions according to a menu provided on display screen50.

It is needless to say that the feature described above is applicable notonly to mobile phones but also to television sets, monitors and thelike. It is preferable that the display screen is provided with iconsfor the user to be capable of immediately recognizing what display statethe current display state is. The matters described above hold true forthe matters to be described below.

The EL display apparatus and the like according to this embodiment areapplicable not only to a digital video camera but also to a digitalstill camera as shown in FIG. 60. The display apparatus is used as amonitor 50 attached to a camera body 601. The camera body 601 is fittedwith a shutter 603 as well as switch 594.

Though the foregoing description is directed to cases where the displayregion of a display panel is relatively small, display screen 50 aslarge as 30 inches or more is likely to warp. To deal with thisinconvenience, the present invention provides the display panel with anouter frame 611 fitted therearound and a fixing member 614 for hangingthe outer frame 611, as shown in FIG. 61. The display panel is fitted onwall or the like by means of this fixing member 614.

However, the weight of the display panel increases with increasingscreen size. For this reason, a leg-mounting portion 613 is providedunder the display panel so that plural legs mounted thereon can supportthe weight of the display panel.

The legs 612 are movable laterally as indicated by arrow A and areexpandable/contractible in directions indicated by arrow B. For thisreason, the display apparatus can be easily installed even in a narrowplace.

A television set shown in FIG. 61 has a screen covered with a protectivefilm (which may be a protective plate.) One object of such coverage isto prevent damage to the surface of the display panel due to a bodyhitting the surface. The protective film has an obverse surface formedwith an AIR coat and embossed to inhibit unwanted reflection of externalscene (extraneous light) by the display panel.

A fixed space is provided between the protective film and the displaypanel by dispersing beads or the like therebetween. Further, theprotective film has a reverse surface formed with fine projections forretaining the space between the display panel and the protective film.By thus retaining the space, an impact is inhibited to transfer from theprotective film to the display panel.

It is also effective to dispose or inject a light coupling agent such asalcohol or ethylene glycol in a liquid state, an acrylic resin in a gelstate, or an epoxy resin which is a solid resin between the protectivefilm and the display panel. This is because interfacial reflection canbe prevented and because the light coupling agent functions also as ashock absorber.

Examples of such protective films include polycarbonate film (plate),polypropylene film (plate), acrylic film (plate), polyester film(plate), and PVA film (plate). It is needless to say that besides thesefilms, engineering resin films (such as ABS) can be used. The protectivefilm may be formed from an inorganic material such as strengthenedglass. A similar effect will be produced if the surface of the displaypanel is coated with epoxy resin, phenolic resin, acrylic resin or thelike to a thickness of not less than 0.5 mm and not more than 2.0 mminstead of the provision of the protective film. Embossing the surfaceof such a resin coat or a like process is also effective.

It is also effective that the surface of the protective film or coatinglayer is coated with fluorine. This is because such a fluorine coatallows stain thereon to be removed easily with a detergent. Theprotective film may be formed thicker so that a front light may sharethe protective film.

It is needless to say that combining the display panel according to theembodiment of the present invention with the three-side-freearrangement. The three-side-free arrangement is effective particularlywhen the pixels are manufactured utilizing the amorphous silicontechnology. With the panel formed utilizing the amorphous silicontechnology, process control for controlling variations in thecharacteristics of transistors is impossible. Hence, it is preferable toapply the N-fold pulse driving method, reset driving method, dummy pixeldriving method or the like according to the present invention to such apanel. Thus, the transistors used in the present invention may be formedby the amorphous silicon technology without limitation to those formedby the polysilicon technology.

The N-fold pulse driving methods (see FIGS. 13, 16, 19, 20, 22, 24 and30 and the like) and like methods according to the present invention areeffective for display panels of the type having transistors 11 formed bythe amorphous silicon technology as well as for display panels of thetype having transistors 11 formed by the low temperature polysilicontechnology. This is because adjacent transistors 11 formed usingamorphous silicon substantially agree to each other in characteristics.Accordingly, driving currents for individual transistors are eachsubstantially equalized to the target value even when the panel isdriven with the sum of currents. (The N-fold pulse driving methodsillustrated in FIGS. 22, 24 and 30 are particularly effective for pixelconfigurations of the type having transistors formed utilizing amorphoussilicon.)

The technical concept described by way of the embodiments of the presentinvention is applicable to digital video cameras, projectors,stereoscopic television, projection television, and the like. Theconcept is also applicable to view finders, mobile phone monitors, PHSs,personal digital assistants and their monitors, and digital stillcameras and their monitors.

Also, the technical concept is applicable to electrophotographicsystems, head-mounted displays, direct viewing monitors, notebook PCsand desktop PCs. Further, the concept is applicable to monitors for cashdispensers, and public telephones, video phones and watches and theirdisplays.

It is needless to say that the technical concept of the presentinvention can be utilized in or applied to development of displaymonitors for household appliances, pocket-size game machines and theirmonitors, back lights for display panels, lighting instruments for homeuse or industrial use, and the like. A lighting instrument is preferablyconfigured to be capable of varying the color temperature. The colortemperature can be varied by adjustment of currents to be fed to R, Gand B pixels if these pixels are arranged in a striped pattern or adot-matrix pattern. The technical concept is also applicable to displayapparatus for displaying advertisements or posters, RGB signals, warningdisplay lights, and the like.

The organic EL display panel is effective as a light source of ascanner. In this case, a dot matrix comprising R, G and B pixels is usedas the light source to illuminate a subject with light in reading theimage of the subject. Of course, it is needless to say that such a lightsource may be designed to emit monochromatic light. Such a light sourcemay be of a simple matrix configuration without limitation to an activematrix configuration. The image reading precision will improve if thecolor temperature can be controlled.

Also, the organic EL display apparatus is effective as the back light ofa liquid crystal display device. The color temperature can be varied byadjustment of currents to be fed to R, G and B pixels of the EL displayapparatus (back light) if these pixels are arranged in a striped patternor a dot-matrix pattern. In this case, the brightness can also becontrolled easily. Moreover, since the EL display apparatus is asurface-emitting light source, it can easily realize a Gaussiandistribution in which a central portion of the screen is made relativelybright whereas a peripheral portion of the screen made relatively dark.The EL display apparatus is also effective as the back light of a liquidcrystal display panel of the field sequential type which performsscanning with R, G and B rays alternately. The EL display apparatus canalso be used as the back light of a liquid crystal display panel or thelike adapted for motion picture display if black is inserted even whenthe back light blinks.

It should be noted that EL device 15 is regarded as an OLED in thepresent invention and represented using the symbol of diode in thedrawings such as FIG. 1. However, EL device 15 according to the presentinvention is not limited to the OLED but may be of any type whichcontrols its luminance based on the amount of current passing through ELdevice 15. An example of such a device is an inorganic EL device. Otherexamples include a white light emitting diode comprising asemiconductor, and a common light-emitting diode. A light-emittingtransistor can serve the purpose. Device 15 does not necessarily callfor rectification. Therefore, device 15 may be a bidirectional diode.

It will be apparent from the foregoing description that manyimprovements and other embodiments of the present invention occur tothose skilled in the art. Therefore, the foregoing description should beconstrued as an illustration only and is provided for the purpose ofteaching the best mode for carrying out the present invention to thoseskilled in the art. The details of the structure and/or the function ofthe present invention can be modified substantially without departingfrom the spirit of the present invention.

INDUSTRIAL APPLICABILITY

The EL display apparatus of the present invention is usable as an imagedisplay unit or the like of a mobile phone.

The driving circuit of the EL display apparatus of the invention isusable as a driving circuit or the like of an image display unit of amobile phone.

The electronic display appliance of the invention is usable as a mobilephone, a television, a display of a personal computer, and so forth.

1. An EL display apparatus comprising: a display screen including aplurality of gate signal lines and a plurality of source signal lineswhich are arranged to intersect with each other, and pixels in each ofwhich an EL light emitting element is formed to correspond to anintersection point of each of the gate signal lines and each of thesource signal lines; a current driving device formed for each of thepixels for driving the EL light emitting element by a second currentresponsive to a source signal represented by a first current; a signalcurrent source for outputting the source signal in response to an imagesignal to the current driving device via the source signal line; and aprecharge voltage source for outputting a predetermined voltage to thesource signal line, wherein the current driving device is configured todrive the EL light emitting element by the second current responsive toa voltage of a control terminal connected to the source signal line; andthe predetermined voltage corresponds to gray scale data of the imagesignal.
 2. The EL display apparatus according to claim 1, furthercomprising a switching and connecting unit capable of selectivelyconnecting either the signal current source or the precharge voltagesource to the source signal line, wherein the switching and connectingunit connects the precharge voltage source and the signal current sourceto the source signal line such that the source signal is output to thesource signal line after the predetermined voltage is applied to thesource signal line in one horizontal scan period.
 3. The EL displayapparatus according to claim 2, wherein a duration of applying thepredetermined voltage is 0.2 μs or more and 3 μs or less.
 4. The ELdisplay apparatus according to claim 2, wherein the current drivingdevice drives the EL light emitting element by the second currentresponsive to the voltage of the control terminal connected to thesource signal line; and the predetermined voltage is a voltage by whichthe current driving device so drives the EL light emitting element as toachieve black display.
 5. The EL display apparatus according to claim 1,further comprising a switching and connecting unit capable ofselectively connecting either the signal current source or the prechargevoltage source to the source signal line, wherein the switching andconnecting unit connects the precharge voltage source to the sourcesignal line when the gray scale data of the image signal is apredetermined one.
 6. The EL display apparatus according to claim 1,wherein a plurality of the EL light emitting elements emitting aplurality of types of colors are connected to a plurality of the sourcesignal lines, respectively, and the precharge voltage source outputs thepredetermined voltages for the colors respectively to the source signallines.
 7. The EL display apparatus according to claim 1, wherein thecurrent driving device comprises a transistor.
 8. The EL displayapparatus according to claim 1, wherein the current driving devicecomprises a current mirror circuit.
 9. The EL display apparatusaccording to claim 1, further comprising a switching and connecting unitcapable of selectively connecting either the signal current source orthe precharge voltage source to the source signal line, wherein aplurality of pixels are disposed in matrix; each of the pixels isprovided with the EL light emitting element and the current drivingdevice; each of columns or rows of the pixels is provided with thesource signal line; the current driving devices of the rows or thecolumns are connected selectably to the source signal lines; each of thesource signal lines is provided with the signal current source, theprecharge voltage source, and the switching and connecting unit; aplurality of gate lines for transmitting a gate signal to select thecurrent driving devices per column or row are provided; and a gatedriver for outputting the gate signal to the gate lines is provided. 10.An electronic display appliance comprising: an image display unitcomprising the EL display apparatus according to claim 1, the EL displayapparatus further comprising a switching and connecting unit capable ofselectively connecting either the signal current source or the prechargevoltage source to the source signal line, wherein a plurality of pixelsare disposed in matrix; each of the pixels is provided with the EL lightemitting element and the current driving device, each of columns or rowsof the pixels is provided with the source signal line, the currentdriving devices of the columns or the rows are connected selectably tothe source signal lines, each of the source signal lines is providedwith the signal current source, the precharge voltage source, and theswitching and connecting unit, a plurality of gate lines fortransmitting a gate signal to select the current driving devices per rowor column are provided, and a gate driver for outputting the gate signalto the gate lines is provided; a receiver; and a speaker.
 11. A drivingcircuit of EL display apparatus, comprising: a plurality of unit currentsources; a reference current generation circuit for defining a currentto be output from the unit current sources; a plurality of currentswitching circuits disposed on output ends of the unit current sources;a current wiring having one end connected to the current switchingcircuits via a first change over switch and the other end connected to asource signal line; and a precharge voltage source which outputs apredetermined voltage and is connected to the current wiring via asecond change over switch, wherein the current switching circuits areturned on and off depending on gray scale data of an image signal, andthe first and the second change over switches selectively connect eitherthe current switching circuits or the precharge voltage source to thesource signal line.
 12. The driving circuit of EL display apparatusaccording to claim 11, wherein the unit current sources are connected tothe current switches in such a fashion that the number of the unitcurrent sources to be aligned parallel and connected to each of thecurrent switches is a number from 2 raised to the 0th power to 2 raisedto the nth power (n is a natural number).
 13. The driving circuit of ELdisplay apparatus according to claim 11, wherein the reference currentgeneration circuit has an operation amplifier, the operation amplifierdefining a current output from the unit current sources.
 14. The ELdisplay apparatus according to claim 1, wherein the precharge voltagesource has an output impedance smaller than an output impedance of thesignal current source.